Background: In the KEYNOTE-177 study, pembrolizumab monotherapy provided statistically significant and clinically meaningful improvements in progression-free survival versus chemotherapy as first-line treatment in patients with microsatellite instability-high or mismatch repair-deficient metastatic colorectal cancer. To further support the efficacy and safety findings of the KEYNOTE-177 study, results of the health-related quality of life (HRQOL) analyses are reported here., Methods: KEYNOTE-177 is an open-label, randomised, phase 3 trial being done at 192 cancer centres in 23 countries, in patients aged 18 years and older with microsatellite instability-high or mismatch repair-deficient metastatic colorectal cancer, with an Eastern Cooperative Oncology Group performance status of 0 or 1, and who had not received previous systemic therapy for metastatic disease. Eligible patients were randomly assigned (1:1) centrally by use of interactive voice response or integrated web response technology to receive pembrolizumab 200 mg intravenously every 3 weeks or investigator's choice chemotherapy (mFOLFOX6 [leucovorin, fluorouracil, and oxaliplatin] or FOLFIRI [leucovorin, fluorouracil, and irinotecan] intravenously every 2 weeks with or without intravenous bevacizumab or cetuximab). Patients and investigators were not masked to treatment assignment. The primary endpoints were progression-free survival (previously reported) and overall survival (data to be reported at the time of the final analysis). HRQOL outcomes were evaluated as prespecified exploratory endpoints. The analysis population comprised all randomly assigned patients who received at least one dose of study treatment and completed at least one HRQOL assessment. HRQOL outcomes were mean change from baseline to prespecified week 18 in European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire Core 30 (EORTC QLQ-C30) and EORTC Quality of Life Questionnaire-Colorectal 29 (EORTC QLQ-CR29) scale and item scores, and in the EuroQoL 5 Dimensions 3 Levels (EQ-5D-3L) visual analogue scale and health utility scores; the proportion of patients with improved, stable, or deteriorated scores from baseline to prespecified week 18 in EORTC QLQ-C30 scales and items; and time to deterioration in EORTC QLQ-C30 global health status/quality of life (GHS/QOL), physical functioning, social functioning, and fatigue scores and EORTC QLQ-CR29 urinary incontinence scores. The threshold for a small and clinically meaningful mean difference in EORTC QLQ-C30 score was 5-8 points. This study is registered with ClinicalTrials.gov, NCT02563002 and is ongoing; recruitment is closed., Findings: Between Feb 11, 2016, and Feb 19, 2018, 307 patients were enrolled and randomly assigned to receive pembrolizumab (n=153) or chemotherapy (n=154). The HRQOL analysis population comprised 294 patients (152 receiving pembrolizumab and 142 receiving chemotherapy). As of Feb 19, 2020, median time from randomisation to data cutoff was 32·4 months (IQR 27·7-37·8). Least squares mean (LSM) change from baseline to prespecified week 18 showed a clinically meaningful improvement in EORTC QLQ-C30 GHS/QOL scores with pembrolizumab versus chemotherapy (between-group LSM difference 8·96 [95% CI 4·24-13·69]; two-sided nominal p=0·0002). Median time to deterioration was longer with pembrolizumab versus chemotherapy for GHS/QOL (hazard ratio 0·61 [95% CI 0·38-0·98]; one-sided nominal p=0·019), physical functioning (0·50 [95% CI 0·32-0·81]; one-sided nominal p=0·0016), social functioning (0·53 [95% CI 0·32-0·87]; one-sided nominal p=0·0050), and fatigue scores (0·48 [95% CI 0·33-0·69]; one-sided nominal p<0·0001)., Interpretation: Pembrolizumab monotherapy led to clinically meaningful improvements in HRQOL compared with chemotherapy in patients with previously untreated microsatellite instability-high or mismatch repair-deficient metastatic colorectal cancer. These data, along with the previously reported clinical benefits, support pembrolizumab as a first-line treatment option for this population., Funding: Merck Sharp & Dohme, a subsidiary of Merck & Co, Kenilworth, NJ, USA., Competing Interests: Declaration of interests TA reports consulting or advisory roles, or both, and received honoraria from Amgen, AstraZeneca, Bristol Myers Squibb, Chugai, Clovis Oncology, GlaxoSmithKline, Gritstone Oncology, HalioDx, Pierre Fabre, Roche/Ventana, Sanofi, Servier, Tesaro and Merck Sharp & Dohme, a subsidiary of Merck & Co, Kenilworth, NJ, USA (MSD); travel, accommodations, and expenses from Roche/Ventana, Bristol Myers Squibb, and MSD; speaker bureau fees from Bristol Myers Squibb and Servier; and research funding from MSD. TA also reports participation in the scientific committee of the ARCAD foundation and GERCOR group as non-renumerated activities. MA is an employee of Merck Sharp & Dohme, a subsidiary of Merck & Co, Kenilworth, NJ, USA, and is a shareholder in Merck & Co, Kenilworth, NJ, USA. JMN is an employee of Merck Sharp & Dohme, a subsidiary of Merck & Co, Kenilworth, NJ USA, and may own stock or hold stock options, or both, in the Company. K-KS reports honoraria from Bristol Myers Squibb, Guardant Health, Innovent Biologics, Merck KGaA, Roche, and Servier; consulting or advisory roles, or both, for Roche; research grants or funding, or both, from Amgen, Bristol Myers Squibb, Gilead, Merck KGaA, Roche, and MSD; and travel, accommodations, and expenses from Merck KGaA, Innovent Biologics, and MSD. TWK reports research grants or funding, or both, from Merck Serono, AstraZeneca, and Pfizer. BVJ reports research grants or funding, or both, from MSD. LHJ reports research grants or funding, or both, to his institution from 2cureX, Incyte, Bristol Myers Squibb, and MSD. CJAP reports advisory roles for Nordic Pharma. DS declares no competing interests. RG-C has provided scientific advice, attended speakers bureau, or received honoraria from AAA, Advanz Pharma, Bayer, Bristol Myers Squibb, HMP, Ipsen, Merck & Co, Midatech Pharma, Novartis, PharmaMar, Pfizer, Pierre Fabre, Roche, Sanofi, Servier, and MSD; and has received support to his institution for the conduct of clinical trials or for molecular diagnostic platforms from ARMO Biosciences, AstraZeneca, Pfizer, Novartis, Ipsen, Roche, Pharmacyclics, Boston Biomedicals, Merck & Co, Amgen, Sanofi, Bayer, Bristol Myers Squibb, Boehringer Ingelheim, Sysmex, Gilead Sciences, Servier, Adacap, VCN, Lilly, Pharmamar, and MSD. RG-C also reports financial support from Pfizer and Bristol Myers Squibb to an investigator-initiated trial evaluating axitinib in neuroendocrine tumours and nivolumab in neuroendocrine carcinomas. RG-C also reports being a member of the Executive Committee of the Spanish Neuroendocrine Tumor Cooperative Group (GETNE), the Executive Committee of the European Society of Neuroendocrine Tumors (ENETS), the Scientific Advisory Group for Oncology (SAG-O) of the European Medicines Agency (EMA; 2008–17), EORTC, ASCO, ESMO, SEOM, TTD, GEMCAD; and is a global PI of a clinical trial of Axitinib (Pfizer) in neuroendocrine tumours and a clinical trial of nivolumab (Bristol Myers Squibb) and chemotherapy in neuroendocrine carcinomas. IS reports advisory or consultancy, or both, roles for Ipsen, Pfizer, Syrtex, Amgen, and Pharmamar; and speaker bureau fees from AAA, Sanofi, and Novartis. CDLF reports advisory or consultancy, or both, roles for Roche, Pierre Fabre, Oncologie, Eisai, Bayer, and MSD; and travel, accommodations, and expenses from Amgen, Eisai, Bristol Myers Squibb, and Roche. FR reports honoraria; advisory or consultancy roles, or both; research grants or funding, or both; travel, accommodations, and expenses from Roche, Merck-Serono, Sanofi, Bristol Myers Squibb, Servier, Lilly, Amgen, Bayer, Celgene, and MSD. EE reports personal financial interests, honoraria, advisory roles, travel grants, and research grants from Hoffman-La Roche, Sanofi, Aventis, Amgen, Merck Serono, Servier, Array Pharmaceuticals, Bristol Myers Squibb, and MSD; and institutional financial interests and honoraria from Hoffman-La Roche, Sanofi Aventis, Amgen, Merck Serono, Boehringer Ingelheim, AbbVie, Array Pharmaceuticals, Pierre Fabre, Novartis, Bristol Myers Squibb, GlaxoSmithKline, MedImmune, and MSD. LAD is a member of the board of directors of Personal Genome Diagnostics and Jounce Therapeutics. LAD is a compensated consultant to PGDx, 4Paws (PetDx), Innovatus CP, Se'er, Kinnate, and Neophore. LAD is an uncompensated consultant to Merck & Co. LAD has received research support for clinical trials from Merck & Co. LAD is an inventor of multiple licensed patents related to technology for circulating tumour DNA analyses and mismatch repair deficiency for diagnosis and therapy from Johns Hopkins University. Some of these licenses and relationships are associated with equity or royalty payments directly to Johns Hopkins and LAD. LAD holds equity in Personal Genome Diagnostics, Jounce Therapeutics, Thrive Earlier Detection, Se'er, Kinnate and Neophore. LAD's spouse holds equity in Amgen. The terms of all these arrangements are being managed by Johns Hopkins and Memorial Sloan Kettering in accordance with their conflict of interest policies. TY reports research funding from Novartis Pharma, MSD, Sumitomo Dainippon Pharma, Chugai Pharmaceutical, Sanofi, Daichi Sankyo, Parexel International, Ono Pharmaceutical, and GlaxoSmithKline. EVC reports advisory board roles for Array, AstraZeneca, Bayer, Biocartis, Bristol Myers Squibb, Celgene, Daiichi Sankyo, Halozyme, GlaxoSmithKline, Incyte, Ipsen, Lilly, Merck KGaA, Novartis, Pierre Fabre, Roche, Servier, Sirtex, Taiho, and MSD; and research funding from Bayer, Boehringer Ingelheim, Celgene, Ipsen, Lilly, Merck KGaA, Novartis, Roche, Servier, and MSD. PY is an employee of MSD China Holding. MF is an employee of Merck Sharp & Dohme, a subsidiary of Merck & Co, Kenilworth, NJ USA, who may own stock or hold stock options, or both in the Company. DTL reports advisory board roles for MSD and Bristol Myers Squibb; research funding from MSD, Bristol Myers Squibb, Aduro Biotech, Curegenix, Medivir, and Nouscom; honoraria from MSD; and is an inventor of licensed intellectual property from Johns Hopkins University., (Copyright © 2021 Elsevier Ltd. All rights reserved.)