Aims: In HER2CLIMB, tucatinib significantly improved progression-free and overall survival in patients with human epidermal growth factor receptor 2-positive (HER2+) metastatic breast cancer. We evaluated the impact of tucatinib on health-related quality of life (HR-QoL) in HER2CLIMB., Methods: Patients were randomised 2:1 to tucatinib or placebo combined with trastuzumab and capecitabine. Starting with protocol version 7, the EuroQol 5 Dimensions 5 Levels (EQ-5D-5L) questionnaire and EQ visual analogue scale (VAS) were administered at day 1 of cycle 1, every two cycles during cycles 3-9, every three cycles during cycle 12 and thereafter and at each patient's 30-day follow-up visit., Results: Among 364 patients eligible for HR-QoL assessment, 331 (91%) completed ≥1 assessment. EQ-VAS scores were similar for both arms at baseline and maintained throughout treatment. EQ-5D-5L scores were similar between the treatment arms, stable throughout therapy and worsened after discontinuing treatment. Risk of meaningful deterioration (≥7 points) on EQ-VAS was reduced 19% in the tucatinib vs. placebo arm (hazard ratio [HR]: 0.81; 95% confidence interval [CI]: 0.55, 1.18); the median (95% CI) time to deterioration was not reached in the tucatinib arm and was 5.8 months (4.3, -) in the placebo arm. Among patients with brain metastases (n = 164), risk of meaningful deterioration on EQ-VAS was reduced 49% in the tucatinib arm (HR: 0.51; 95% CI: 0.28, 0.93); the median (95% CI) time to deterioration was not reached in the tucatinib arm and was 5.5 months (4.2, -) in the placebo arm., Conclusions: HR-QoL was preserved for patients with HER2+ metastatic breast cancer who were treated with tucatinib added to trastuzumab and capecitabine and maintained longer with tucatinib therapy than without it among those with brain metastases., Clinical Trial Registration: NCT02614794., Competing Interests: Conflict of interest statement V.M. reports consultancy for Amgen, AstraZeneca, ClinSol, Daiichi Sankyo, Eisai, Genomic Health, Hexal, Lilly, MSD Oncology, Novartis, Pierre Fabre, Roche, Seagen, Tesaro and Teva and research funding/grants from Novartis, Roche/Genentech and Seagen. A.W. reports corporate board membership for Andrew Wardley Limited, Manchester Cancer Academy and Outreach Research & Innovation Group Limited; consultancy for Accord Research, AstraZeneca, Athenex, Coleman Expert Network, Coleman Research, Daiichi Sankyo, Gerson Lehrman Group, Guidepoint Global, Lilly, MSD Oncology, NAPP Pharmaceuticals, Novartis, Pfizer and Roche; employment at the Christie NHS Foundation Trust and AstraZeneca (as of 11th January 2021); patents and royalties for Outreach Research & Innovation Group; speaker's bureau participation for AstraZeneca, Eisai, Lilly, Novartis, Pfizer and Roche; travel expenses from Daiichi Sankyo, MSD and Roche and other nonfinancial relationships with the Strategy Director for the Association of Cancer Physicians, Committee Member UK Breast Cancer Group, Committee Member NHS England and Chemo Clinical Reference Group, ESMO Breast Cancer Faculty and NCRI Breast Research Group Chair. E.P. reports consultancy for Mylan, Novartis, Pfizer and R-Pharm; research funding/grants from AbbVie, Cascadian, Corcept Therapeutics, Genentech, Hoosier Cancer Research Network, Merck, Novartis and Seagen and travel expenses from Amgen, Genentech, Merck, Novartis and Tesaro. E.H. reports consultancy for AstraZeneca, Black Diamond Therapeutics, Boehringer Ingelheim, Daiichi Sankyo, Genentech/Roche, Lilly, Mersana, Novartis, Pfizer, Puma Biotechnology and Silverback Therapeutics; research funding/grants from AbbVie, AceraPharma, Aravive, ArQule, Arvinas, AstraZeneca, BerGenBio, Black Diamond Therapeutics, Boehringer, Clovis Oncology, Compugen, Curis, CytomX Therapeutics, Daiichi Sankyo, Deciphera, eFFECTOR, Eisai, EMD Serono, Fochon Pharmaceuticals, Fosun Orinove, Fujifilm, G1 Therapeutics, Pfizer, Puma Biotechnology, Radius Health, Regeneron, Rgenix, Seagen, Sermonix Pharmaceuticals, Silverback Therapeutics, Stemcentrx, Sutro Biopharma, Syndax, Syros Pharmaceuticals, Taiho Pharmaceutical, Takeda, TapImmune, Tesaro, Torque, Unum Therapeutics, Verastem, Zenith Epigenetics, Zymeworks, Genentech/Roche, H3 Biomedicine, Harpoon, Hutchison MediPharma, Immunogenics, InventisBio, Karyopharm Therapeutics, Leap Therapeutics, Lilly, Lycera, MacroGenics, MedImmune, Medivation, Mersana, Merus, Millennium, Molecular Templates, Novartis, NuCana and OncoMed and travel expenses from Amgen, AstraZeneca, Bayer, BMS, Clovis, Eisai, EMD Serono, Foundation Medicine, Genentech/Roche, Genentech, Genzyme, Guardant Health, Helsinn Therapeutics, Heron, Lilly, Medivation, Merck, Novartis, Pfizer, Roche, Sysmex and Tesaro. A.Z. reports consultancy for Norvatis and Pfizer and travel expenses from Immunomedics. E.J. reports consultancy for Lilly, Novartis, Pfizer and Roche. F.B. reports consultancy and honoraria from Lilly, Novartis, Lilly and Roche and travel expenses from Novartis. E.H.B. reports travel expenses from Pierre Fabre, Pfizer and Roche. A.B. reports corporate board membership for NanoTx Therapeutics; consultancy for AlaMab Therapeutics, NanoTx Therapeutics, Plus Therapeutics and Vascular Biogenics; equity ownership in NanoTx and Plus Therapeutics; honoraria from Vascular Biogenics; patents and royalties related to intellectual property interest in NanoTx Therapeutics; research funding/grants from Boston Biomedical, Immunomedics, Medicenna, Mirna Therapeutics, Threshold Pharmaceuticals, Upsher-Smith and Vascular Biogenics and travel expenses from Vascular Biogenics. L.C. reports speaker's bureau participation for Astellas, Ipsen and Janssen and other fees for Astellas, Bristol Myers Squibb, Ipsen, Novartis and Pfizer. C.F. reports honoraria from Pfizer, Bayer, Novartis, AstraZeneca, Merck, Astellas Pharma and Roche Canada; a consulting or advisory role for Genomic Health, Merck, AstraZeneca, Bayer and Odonate Therapeutics; speaker's bureau for Merck; research funding/grants to self from Bayer; research funding/grant to the institution from Astellas Pharma, AstraZeneca, Lilly, Merck, Novartis, Roche/Genentech, Sanofi, Pfizer, Janssen Oncology, Zymeworks, Seagen, Immunomedics, Bicycle Therapeutics and Sermonix Pharmaceuticals and travel expenses from Novartis and Roche. M.M.-M. reports advisory board membership for Pierre Fabre; grants for conference attendance from Roche, Pfizer and Lilly and expert testimony for Novartis, Roche and Eisai. H.T.A. reports advisory board membership for Bayer, BeiGene, Bicycle, BioNTech, iOnctura, Roche and Servier and employment at HCA Healthcare UK and Sarah Cannon. N.I. reports consultancy for Janssen, Merck, Novartis and Pfizer. S.A. reports speaker's bureau participation for Novartis, Pfizer, Puma, Daiichi/AstraZeneca, Merck and Seagen. M.C. reports consultancy for Sanofi and travel expenses from Novartis, Bristol Myers Squibb and Ipsen. T.P. reports research funding/grants from Merck, Pfizer and Seagen. E.S.-R. reports consultancy for Lilly and Mylan, research funding/grants from Susan G. Komen Victory Foundation and speaker's bureau participation for Mylan. D.C. reports consultancy for Daiichi Sankyo, GlaxoSmithKline, Novartis, Roche and Synthon; employment at Edinburgh Cancer Research Centre, Edinburgh, United Kingdom and research funding/grants from Daiichi Sankyo, GlaxoSmithKline, Novartis, Roche and Seagen (all funding is to his institution). G.C. reports consultancy from Bristol Myers Squibb, Lilly, Novartis, Pfizer, Roche, Seagen and Daiichi Sankyo; employment at the University of Milano, Istituto Europeo di Oncologia, IRCCS, Milano, Italy and speaker's bureau participation for Lilly, Pfizer, Roche, Seagen and Daiichi Sankyo. K.G. reports consultancy for AstraZeneca, Bristol Myers Squibb, Genentech, Genomic Health, Janssen, Lilly, Merck, Mylan, NanoString, Novartis, Pfizer and Roche and research funding/grants from AstraZeneca, Bristol Myers Squibb, Pfizer and Roche. M.S. reports employment at Seagen Inc. and equity ownership in Seagen Inc. and Moderna Pharma. K.D. reports employment at Seagen Inc. and equity ownership in Seagen Inc. and Roche. J.R. and W.F. report employment at Seagen Inc. and equity ownership in Seagen Inc. M.B., S.C., M.C., E.C., L.F. and O.H. have declared no conflicts of interest., (Copyright © 2021 The Authors. Published by Elsevier Ltd.. All rights reserved.)