Background: CheckMate 8HW prespecified dual primary endpoints, assessed in patients with centrally confirmed microsatellite instability-high or mismatch repair-deficient status: progression-free survival with nivolumab plus ipilimumab compared with chemotherapy as first-line therapy and progression-free survival with nivolumab plus ipilimumab compared with nivolumab alone, regardless of previous systemic treatment for metastatic disease. In our previous report, nivolumab plus ipilimumab showed superior progression-free survival versus chemotherapy in first-line microsatellite instability-high or mismatch repair-deficient metastatic colorectal cancer in the CheckMate 8HW trial. Here, we report results from the prespecified interim analysis for the other primary endpoint of progression-free survival for nivolumab plus ipilimumab versus nivolumab across all treatment lines., Methods: CheckMate 8HW is a randomised, open-label, international, phase 3 trial at 128 hospitals and cancer centres across 23 countries. Immunotherapy-naive adults with unresectable or metastatic colorectal cancer across different lines of therapy and microsatellite instability-high or mismatch repair-deficient status per local testing were randomly assigned (2:2:1) to nivolumab plus ipilimumab (nivolumab 240 mg, ipilimumab 1 mg/kg, every 3 weeks for four doses; then nivolumab 480 mg every 4 weeks; all intravenously), nivolumab (240 mg every 2 weeks for six doses, then 480 mg every 4 weeks; all intravenously), or chemotherapy with or without targeted therapies. The dual independent primary endpoints were progression-free survival by blinded independent central review with nivolumab plus ipilimumab versus chemotherapy (first line) and progression-free survival by blinded independent central review with nivolumab plus ipilimumab versus nivolumab (all lines) in patients with centrally confirmed microsatellite instability-high or mismatch repair-deficient metastatic colorectal cancer. This study is registered with ClinicalTrials.gov (NCT04008030)., Findings: Between Aug 16, 2019, and April 10, 2023, 707 patients were randomly assigned to nivolumab plus ipilimumab (n=354) or nivolumab alone (n=353). 296 (84%) of 354 patients in the nivolumab plus ipilimumab group and 286 (81%) of 353 patients in the nivolumab group were centrally confirmed to have microsatellite instability-high or mismatch repair-deficient status. At the data cutoff on Aug 28, 2024, median follow-up (from randomisation to data cutoff) was 47·0 months (IQR 38·4 to 53·2). Nivolumab plus ipilimumab treatment showed significant and clinically meaningful improvement in progression-free survival versus nivolumab (hazard ratio 0·62, 95% CI 0·48-0·81; p=0·0003). Median progression-free survival was not reached with nivolumab plus ipilimumab (95% CI 53·8 to not estimable) and was 39·3 months with nivolumab (22·1 to not estimable). Treatment-related adverse events of any grade occurred in 285 (81%) of 352 patients receiving nivolumab plus ipilimumab and in 249 (71%) of 351 patients receiving nivolumab; grade 3 or 4 treatment-related adverse events occurred in 78 (22%) and 50 (14%) patients, respectively. There were three treatment-related deaths: one event of myocarditis and pneumonitis each in the nivolumab plus ipilimumab group and one pneumonitis event in the nivolumab group., Interpretation: Nivolumab plus ipilimumab showed superior progression-free survival versus nivolumab across all treatment lines, with a manageable safety profile, in patients with microsatellite instability-high or mismatch repair-deficient metastatic colorectal cancer. These results, together with the first-line results of superior progression-free survival with nivolumab plus ipilimumab versus chemotherapy, suggest nivolumab plus ipilimumab as a potential new standard of care for patients with microsatellite instability-high or mismatch repair-deficient metastatic colorectal cancer., Funding: Bristol Myers Squibb and Ono Pharmaceutical., Competing Interests: Declaration of interests TA reports consulting or advisory roles for Amgen, Aptitude Health, Astellas Pharma, Bristol Myers Squibb, GamaMabs Pharma, Gritstone Bio, Gilead Sciences, GlaxoSmithKline, MSD Oncology, Nordic Bioscience, Seagen, Servier, Pfizer, Pierre Fabre, Takeda-Lundbeck, Tesaro, and Transgene; receiving honoraria from Amgen, Bristol Myers Squibb, MSD Oncology, Merck Serono, GlaxoSmithKline, Pierre Fabre, Roche-Genentech, Sanofi, Seagen, Servier, and Ventana Medical Systems; a data monitoring committee role for Inspirna; being President of the ARCAD foundation (Aide à la recherché en cancérologie digestive), a member of the ACCENT Collaborative Group and Gercor; and travel or accommodation expenses from Bristol Myers Squibb, MSD Oncology, Takeda, and Servier. EE reports consulting or advisory roles for Amgen, Bayer, Boehringer Ingelheim, Bristol Myers Squibb, Cureteq, Jannsen, Merck Serono, Merck Sharp & Dohme, Novartis, Pfizer, Pierre Fabre, Repare Therapeutics, RIN Institute, Roche-Genentech, Sanofi, Seagen, Servier, and Takeda; receiving honoraria from Amgen, Bayer, Bristol Myers Squibb, Boehringer Ingelheim, Cureteq, Janssen, Lilly, Medscape, Merck Sharp & Dohme, Merck Serono, Novartis, Pfizer, Pierre Fabre, Repare Therapeutics, RIN Institute, Roche-Genentech, Sanofi, Seagen, Servier, and Takeda; research funding to their institution from AbbVie, Amgen, Array BioPharma, AstraZeneca, Bayer, BeiGene, Bioncotech, BioNTech, Boehringer Ingelheim, Boehringer Ingelheim (Spain), Bristol Myers Squibb, Celgene, Daiichi Sankyo, Debiopharm Group, Gercor, HalioDx, Hutchison MediPharma, Iovance Biotherapeutics, Janssen-Cilag, Janssen R&D, MedImmune, Menarini, Merck, Merck Sharp & Dohme, Merus NV, Mirati, Novartis, Nouscom, PharmaMar, Pfizer, PledPharma, RedX Pharma, Pierre Fabre, Roche-Genentech, Sanofi, Scandion Oncology, Seagen, Servier, Sotio, Taiho, and WntResearch; and travel or accommodation expenses from Amgen, Bayer, Boehringer Ingelheim, Bristol Myers Squibb, Cureteq, Jannsen, Lilly, Medscape, Merck Sharp & Dohme, Merck Serono, Novartis, Pfizer, Pierre Fabre, Repare Therapeutics, RIN Institute, Roche-Genentech, Sanofi, Servier, Seagen, and Takeda. H-JL reports consulting or advisory roles for Bayer, Bristol Myers Squibb, Fulgent Genetics, GlaxoSmithKline, Merck Serono, Roche-Genentech, and 3T BioSciences; receiving honoraria from Bayer, Boehringer Ingelheim, Fulgent Genetics, G1 Therapeutics, Isofol Medical, Jazz Pharmaceuticals, Merck Serono, Oncocyte, and Roche-Genentech; and travel or accommodation expenses from Bayer, Bristol Myers Squibb, and Merck Serono. LHJ reports receiving research grants to their institution from 2cureX, Bristol Myers Squibb, Roche-Genentech, Incyte, Merck Sharp & Dohme, and Pfizer. YT reports consulting or advisory roles for Bristol Myers Squibb, Merck Serono, Pierre Fabre, and Servier; and travel or accommodation expenses from Merck Serono, MSD Oncology, Pierre Fabre, and Servier. EVC reports consulting or advisory roles for AbbVie, Agenus, Amgen, ALX Oncology, Arcus Biosciences, Astellas Pharma, AstraZeneca, Bayer, BeiGene, Boehringer Ingelheim, Bristol Myers Squibb, Daiichi Sankyo, Debiopharm Group, ElmediX, Eisai, GlaxoSmithKline, Hookipa Biotech, Incyte, Ipsen, Lilly, Merck KGaA, Merck Sharp & Dohme, Mirati, Nordic Group, Novartis, Pfizer-BioNTech, Pierre Fabre, Roche-Genentech, Seagen, Servier, Simcere, Taiho, Takeda, and Terumo. RG-C reports receiving honoraria from Advanced Accelerator Applications-Novartis, Astellas Pharma, AstraZeneca, Bayer, Boehringer Ingelheim, Bristol Myers Squibb, Esteve, Gilead Sciences, Hutchmed, Ipsen, Lilly, Merck, Merck Sharp & Dohme, Midatech Pharma, Mirati, Pfizer, Pharmamar, Roche-Genentech, Sanofi, Servier, and Takeda; receiving funding to their institution from Bristol Myers Squibb, Merck Sharp & Dohme, and Pfizer; and travel or accommodation expenses from Advanced Accelerator Applications-Novartis, Esteve, Ipsen, Merck Serono, and Merck Sharp & Dohme. DT reports consulting or advisory roles for AstraZeneca, Bristol Myers Squibb, Merck Sharp & Dohme, Novartis, Pierre Fabre, Sanofi, Servier, and Takeda; receiving honoraria from Amgen, AstraZeneca, Bristol Myers Squibb, Merck Serono, Merck Sharp & Dohme, Pierre Fabre, Roche-Genentech, Sanofi, Servier-Pfizer, and Takeda; research funding to their institution from BTG, Merck Sharp & Dohme, Pierre Fabre, Roche-Genentech, and Takeda; and travel or accommodation expenses from Amgen, Bristol Myers Squibb, MSD Oncology, Pierre Fabre, Roche-Genentech, and Servier. GAM reports consulting or advisory roles for Amgen, Bristol Myers Squibb, Merck KGaA, Merck Sharp & Dohme, Pfizer, and Roche-Genentech; receiving speakers fees from Amgen, Bayer, Bristol Myers Squibb, Grupo Biotoscana, Merck KGaA, Merck Sharp & Dohme, Pfizer, Roche-Genentech, and Servier; research funding from Amgen, AstraZeneca, Bristol Myers Squibb, Merck Sharp & Dohme, and Roche-Genentech; patents, royalties, and intellectual properties with Bristol Myers Squibb and Merck Sharp & Dohme; and travel or accommodation expenses from Amgen, Grupo Biotoscana, Merck KGaA, Pfizer, and Servier. MS reports travel or accommodation expenses from Bristol Myers Squibb and Pfizer; and research funding from AbbVie, Amgen, Astellas Pharma, AstraZeneca, BeiGene, Bioven, Bristol Myers Squibb, Clovis Oncology, Daiichi Sankyo Europe, Eisai, Five Prime Therapeutics, Gilead Sciences, GlaxoSmithKline, Lilly, Merck Sharp & Dohme, Mylan, Novartis, Pfizer-EMD Serono, PharmaMar, Regeneron, Roche, Tesaro, Samsung Healthcare, and Sanofi-Regeneron. CdlF reports consulting or advisory roles for Amgen, Astellas Pharma, Bristol Myers Squibb, Daiichi Sankyo, Eisai, Ipsen, Lilly, MSD Oncology, Pierre Fabre, Roche-Genentech, Servier, and Takeda; research funding to their institution from Merck Sharp & Dohme, Pierre Fabre, and Servier; and travel or accommodation expenses from Amgen, MSD Oncology, Pierre Fabre, Roche-Genentech, and Servier. TY reports consulting or advisory roles for Sumitomo Corporation; receiving honoraria from Chugai Pharma, Merck KGaA, Merck Sharp & Dohme, and Takeda; and research funding to their institution from Amgen, Bristol Myers Squibb Japan, Chugai Pharma, Daiichi Sankyo, Eisai, Falco Biosystems, Medical & Biological Laboratories Co, Merck Sharp & Dohme, Merus, Molecular Health, Ono Pharmaceutical, Pfizer, Roche-Genentech, Sanofi, Sysmex, Taiho, and Takeda. GT reports consulting or advisory roles for AstraZeneca, Bristol Myers Squibb, Merck KGaA, and Servier. MC reports consulting or advisory roles for Bristol Myers Squibb and Numab; and research funding to their institution from Bristol Myers Squibb, Merck Sharp and Dohme, and Roche-Genentech. EG reports consultancy roles for AstraZeneca, Bristol Myers Squibb, Daiichi Sankyo, and Merck Sharp and Dohme; and travel or accommodation expenses from Daiichi Sankyo. MIB reports consulting or advisory roles for Astellas, AstraZeneca, Bristol Myers Squibb, Daiichi Sankyo, Ipsen, Merck Sharp & Dohme, and Pfizer; receiving honoraria from Astellas, AstraZeneca, Bristol Myers Squibb, Daiichi Sankyo, Ipsen, Merck Sharp & Dohme, Pfizer, and Servier; and travel or accommodation expenses from Astellas and Daiichi Sankyo. RJ reports consulting or advisory roles for AstraZeneca, Merck Sharp & Dohme, and Roche-Genentech; receiving honoraria from Bristol Myers Squibb, Gilead Sciences, Ipsen, Merck Sharp & Dohme, Pfizer, Pfizer-EMD Serono, and Roche-Genentech; receiving speakers fees from Gilead Sciences; and travel or accommodation expenses from Novartis. FA reports consulting or advisory roles for Amgen, Bristol Myers Squibb, Merck KGaA, Pfizer, and Taiho; receiving honoraria from Amgen, Bristol Myers Squibb, Merck KGaA, Pfizer, and Taiho; and research funding to their institution from Bristol Myers Squibb, GlaxoSmithKline, Merck KGaA, and Novartis. EC, TCh, ML, LJ, and SIB report employment with Bristol Myers Squibb and ownership of stock in Bristol Myers Squibb. SL reports consulting or advisory roles for Amgen, Astellas Pharma, AstraZeneca, Bayer, Bristol Myers Squibb, Daiichi Sankyo, GlaxoSmithKline, Incyte, Lilly, Merck Serono, Merck Sharp & Dohme, Servier, Rottapharm Biotech, and Takeda; receiving speakers fees from Amgen, Bristol Myers Squibb, GlaxoSmithKline, Incyte, Lilly, Merck Serono, MSD Oncology, Pierre Fabre, Roche-Genentech, and Servier; and research funding from Amgen and Merck Serono (to self), and from AstraZeneca, Bayer, Bristol Myers Squibb, Lilly, and Roche-Genentech (to their institution). All other authors declare no competing interests., (Copyright © 2025 Elsevier Ltd. All rights reserved, including those for text and data mining, AI training, and similar technologies.)