Background: In the European Organisation for Research and Treatment of Cancer (EORTC) 1325-MG/KEYNOTE-054 study, adjuvant pembrolizumab improved recurrence-free survival and distant-metastasis-free survival in patients with resected stage III melanoma. Earlier results showed no effect of pembrolizumab on health-related quality of life (HRQOL). Little is known about HRQOL after completion of treatment with pembrolizumab, an important research area concerning patients who are likely to become long-term survivors. This study reports long-term HRQOL results., Methods: This double-blind, randomised, controlled, phase 3 trial compared adjuvant pembrolizumab with placebo in patients aged 18 years or older with previously untreated stage IIIA, IIIB, or IIIC resected cutaneous melanoma and an Eastern Cooperative Oncology Group performance status score of 1 or 0, recruited from 123 academic centres and community hospitals in 23 countries. Patients were randomly assigned (1:1) with a minimisation technique stratified for stage and geographical region to receive 200 mg of intravenous pembrolizumab or placebo every 3 weeks for up to 18 doses. Investigators, patients, and those collecting or analysing data were masked to group assignment. The primary endpoint of the trial was recurrence-free survival (reported elsewhere). HRQOL was a prespecified exploratory endpoint, measured with the EORTC Quality of Life Questionnaire-Core 30. All patients with a baseline HRQOL evaluation available who were alive 108 weeks from randomisation were included in this analysis of long-term HRQOL. Long-term HRQOL included assessments measured every 6 months between 108 weeks and 48 months after randomisation. The threshold of clinical relevance for all HRQOL scales used was an average change of 5 points. The trial is ongoing, recruitment is completed, and HRQOL data collection is finalised. This study is registered with ClinicalTrials.gov, NCT02362594, and EudraCT, 2014-004944-37., Findings: Between Aug 26, 2015, and Nov 14, 2016, 1019 patients were randomly assigned to pembrolizumab (n=514) or placebo (n=505). Completion of the HRQOL evaluation at baseline exceeded 90% (481 [94%] patients in the pembrolizumab group and 467 [92%] in the placebo group), and ranged between 60% and 90% for post-baseline timepoints. Among patients with a baseline HRQOL evaluation, 365 (39%) were female and 583 (61%) were male. The mean change from baseline to long-term HRQOL was -0·56 (95% CI -2·33 to 1·22) in the pembrolizumab group and 1·63 (-0·12 to 3·38) in the placebo group. The difference between the two groups was -2·19 (-4·65 to 0·27, p=0·081). Differences for all other scales were smaller than 5 and not statistically significant., Interpretation: Adjuvant pembrolizumab did not have a significant impact on long-term HRQOL compared with placebo in patients with resected stage III melanoma. These findings, together with earlier results on efficacy and HRQOL, support the use of pembrolizumab in this setting., Funding: Merck Sharp & Dohme., Competing Interests: Declaration of interests EB's spouse and daughter hold shares in Sandoz, Galenica, Alcon, Roche, and Novartis. MK reports study funding paid to the institution by Merck Sharp & Dohme (MSD) since the initial planning of the work, and study funding paid to the institution from Bristol Myers Squibb (BMS), Immunocore, Johnson & Johnson, and Pierre Fabre in the past 36 months. MM reports payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from BMS, MSD, Novartis, Pierre Fabre, Sanofi, and Sun Pharma. GVL reports consulting fees from Agenus, Amgen, Array Biopharma, AstraZeneca, Bayer, BioNTech, Boehringer Ingelheim, BMS, Evaxion, Hexal AG (Sandoz Company), Highlight Therapeutics, IOBiotech, Immunocore, Innovent Biologics USA, MSD, Novartis, PHMR, Pierre Fabre, Regeneron, Scancell, and SkylineDX BV. VA reports payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events from BMS, MSD, Novartis; support from BMS for attending meetings or travel; participation on a data safety monitoring board or advisory board for BMS, MSD, Novartis, and Immunocore; CUB reports personal fees from MSD, grants and personal fees from Novartis and BMS, personal fees from Roche, GSK, AstraZeneca, Pfizer, Lilly, GenMAB, Pierre Fabre, and Third Rock ventures, grants from NanoString and 4SC, during the conduct of the study. CUB has patents pending (WO 2021/177822 A1 and WO 2023/022596 A1), and reports stock ownership: co-founder of Immagene BV and Signature Oncology. AH reports payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from BMS, MSD, and Novartis; payments for participation on a data safety monitoring board or advisory board with BMS, MSD, and Novartis. SD reports research funding from BMS, MSD, and Pierre Fabre to the institution; payment or honoraria to the institution for lectures, presentations, speakers bureaus, manuscript writing, or educational events from BMS and MSD; support for attending meetings or travel from BMS and MSD; trim 24 patent pending; participation on a data safety monitoring board or advisory board at BMS and MSD, with payments made to the institution; spouse being a Sanofi employee. AK reports consulting fees for an advisory role at Moderna and speaker honorarium from MSD. MSC reports honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events from BMS, MSD and Novartis; having served on advisory boards for Amgen, BMS, Eisai, Ideaya, MSD, Nektar, Novartis, Oncosec, Pierre-Fabre, Qbiotics, Regeneron, Roche, Merck, and Sanofi. AM reports support of scientific and educational conference from Janssen. SS reports grants or contracts from Novartis/Advanced Accelerator Applications (AAA), AstraZeneca, MSD, Genentech, Senhwa, and Pfizer; funding goes to the institution to undertake investigator initiated clinical trial and translational research. SS reports participation on advisory boards for MSD, BMS, AstraZeneca—fees for attending go to research funds at the institution. SS is the chair of DSMB for two Novartis/AAA sponsored phase 3 trials—no fee accepted for chair role. SP reports payment or honoraria to her and her institution for lectures, presentations, speakers bureaus, manuscript writing, or educational events from BMS, MSD, and Novartis, and to her partner and her institution from Amgen and Phylogen; payments to her institution for participation on a data safety monitoring board or advisory board with MSD; leadership or fiduciary role in other board, society, committee, or advocacy group, paid or unpaid from GEM, EORTC melanoma group, and ASEICA. DS reports research grants to his institution from Amgen, Array/Pfizer, BMS, MSD, Novartis, and Roche; consulting fees paid personally from 4SC, Amgen, Array Biopharma, AstraZeneca, BMS, Daiichi Sankyo, Haystack, Immunocore, InFlarX, Innovent, Labcorp, Merck Serono, MSD, Nektar, Neracare, Novartis OncoSec, Pfizer, Philogen, Pierre Fabre, Replimune, Roche, Sandoz, Sanofi/Regeneron, and Sun Pharma; personal payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from BMS, MSD, Merck Serono, Novartis, Roche, Sanofi, and SunPharma; personal support for attending meetings or travel from BMS, Merck Serono, MSD, Novartis, Sanofi, and Pierre Fabre; personal payments for participation on a data safety monitoring board or advisory board at 4SC, Amgen, Array Biopharma, AstraZeneca, BMS, Daiichi Sankyo, Haystack, Immunocore, InFlarX, Merck Serono, MSD, Nektar, Neracare, Novartis, OncoSec, Pfizer, Philogen, Pierre Fabre, Replimune, Roche, Sandoz, Sanofi/Regeneron, and Sun Pharma; leadership or fiduciary role, paid or unpaid, at Dermatologic Cooperative Oncology Group, German Cancer Society, Hiege Stiftung, Deutsche Hautkrebsstiftung, Nationale Versorgungskonferenz Hautkrebs eV, and European Melanoma Registry; drug supply (nivolumab, ipilimumab) from BMS. RJ reports research funding from Iovance Biotherapeutics; honoraria for advisory role or speaker presentation from Merck, BMS, Medison, Pfizer, and Novartis. PR reports consulting fees paid to himself from MBS, MSD, Novartis, Pierre Fabre, Philogen, and Pfizer; honoraria for lectures, presentations, speakers bureau, manuscript writing, or educational events paid to himself from BMS, MSD, Novartis, Pfizer, Pierre Fabre, Sanofi, MSD, and AstraZeneca; speakers bureau Pfizer, Novartis, Pierre Fabre, MSD, and BMS paid to himself; support for attending meetings or travel from Orphan Europe and Pierre Fabre paid to himself; research funding to his institution from Novarits, Pfizer, Roche, and BMS. AJMvdE reports study grants from BMS and Idera; travel expenses from Ipsen; advisory board from BMS, MSD Oncology, Ipsen, Pierre Fabre, and Janssen Cilag BV. CC reports support for attending meetings or travel from Orphan Europe and Pierre Fabre. DG reports stock or stock options from Merck & Co. CK reports stock or stock options from Merck & Co and being an employee of Merck & Co. CR reports consulting fees (payments made to her) from BMS, Roche, Pierre Fabre, Novartis, Sanofi, Pfizer, MSD, Merck, Sunpharma, Ultimovacs, Regeneron, Egle, Philogen, Maat Pharma, and IO Biontech; payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events (payments made to her) from Pierre Fabre, Sanofi, BMS, MSD, and Novartis; support for attending meetings or travel from Pierre Fabre; participation on a data safety monitoring board or advisory board (payments made to her) with BMS, Roche, Pierre Fabre, Novartis, Sanofi, Pfizer, MSD, Merck, Sunpharma, Ultimovacs, Regeneron, Egle, Philogen, and Maat Pharma. AMME reports honoraria for scientific advisory board or independent data monitoring committee functions from Agenus, BioInvent, BioNTech, Boehringer Ingelheim, Brenus, CatalYm, Ellipses, EikonTX, Eurobio, Galecto, IO Biotech, IQVIQ, ISA Pharmaceuticals, Merck & Co, MSD, Pfizer, Pierre Fabre, Scorpion TX, Sairopa, Sellas, SkylineDX, TigaTx, and Trained Therapeutics Discovery; honoraria from Acetra, GenOway, GSK, Moderna, and Trained Immunity Tx; equity from IO Biotech, Sairopa, and SkylineDx; being the Editor in Chief of the European Journal of Cancer. All other authors declare no competing interests., (Copyright © 2024 Elsevier Ltd. All rights reserved, including those for text and data mining, AI training, and similar technologies.)