Background: Pembrolizumab has shown efficacy in persistent, recurrent, or metastatic cervical cancer. The effect of chemoradiotherapy might be enhanced by immunotherapy. In this phase 3 trial, we assessed the efficacy and safety of adding pembrolizumab to chemoradiotherapy in locally advanced cervical cancer., Methods: In this randomised, double-blind, placebo-controlled, phase 3 ENGOT-cx11/GOG-3047/KEYNOTE-A18 clinical trial, adults (age ≥18 years) at 176 medical centres in 30 countries with newly diagnosed, high-risk, locally advanced cervical cancer were randomly assigned (1:1) using an interactive voice-response system with integrated web response to receive 5 cycles of pembrolizumab (200 mg) or placebo every 3 weeks plus chemoradiotherapy, followed by 15 cycles of pembrolizumab (400 mg) or placebo every 6 weeks. Randomisation was stratified by planned external beam radiotherapy type (intensity-modulated radiotherapy or volumetric-modulated arc therapy vs non-intensity-modulated radiotherapy or non-volumetric-modulated arc therapy), cervical cancer stage at screening (International Federation of Gynecology and Obstetrics 2014 stage IB2-IIB node positive vs stage III-IVA), and planned total radiotherapy (external beam radiotherapy plus brachytherapy) dose (<70 Gy vs ≥70 Gy equivalent dose in 2 Gy fractions). Primary endpoints were progression-free survival per Response Evaluation Criteria in Solid Tumours version 1.1-by investigator or by histopathologic confirmation of suspected disease progression-and overall survival. Primary analysis was conducted in the intention-to-treat population, which included all randomly allocated participants. Safety was assessed in the as-treated population, which included all randomly allocated patients who received at least one dose of study treatment. This study is registered with ClinicalTrials.gov, NCT04221945, and is closed to new participants., Findings: Between June 9, 2020, and Dec 15, 2022, 1060 participants were randomly assigned to treatment, with 529 assigned to the pembrolizumab-chemoradiotherapy group and 531 to the placebo-chemoradiotherapy group. At data cutoff (Jan 9, 2023), median follow-up was 17·9 months (IQR 11·3-22·3) in both treatment groups. Median progression-free survival was not reached in either group; rates at 24 months were 68% in the pembrolizumab-chemoradiotherapy group versus 57% in the placebo-chemoradiotherapy group. The hazard ratio (HR) for disease progression or death was 0·70 (95% CI 0·55-0·89, p=0·0020), meeting the protocol-specified primary objective. Overall survival at 24 months was 87% in the pembrolizumab-chemoradiotherapy group and 81% in the placebo-chemoradiotherapy group (information fraction 42·9%). The HR for death was 0·73 (0·49-1·07); these data have not crossed the boundary of statistical significance. Grade 3 or higher adverse event rates were 75% in the pembrolizumab-chemoradiotherapy group and 69% in the placebo-chemoradiotherapy group., Interpretation: Pembrolizumab plus chemoradiotherapy significantly improved progression-free survival in patients with newly diagnosed, high-risk, locally advanced cervical cancer., Funding: Merck Sharp & Dohme, a subsidiary of Merck & Co (MSD)., Competing Interests: Declaration of interests DL, YX, KH, GS, ML, PR-E, AAc, VSu, NC, AJPdSG, FCM, AR, AAy, J-YL, VSa, FZ, LG, JS, ET, KLin, RLaz, C-LC, RLam, HZ, AO, MC, SPo, TU, SPi, and LRD received funding to their institutions from MSD to support the study. DL reports consulting fees from AstraZeneca, Clovis Oncology, GSK, MSD, ImmunoGen, Genmab, Seagen, Novartis, and PharmaMar; payment or honoraria for lectures, presentations, speakers' bureaus, manuscript writing, or educational events from AstraZeneca, ImmunoGen, GSK, Seagen, and Genmab; support for attending meetings or travel from AstraZeneca, Clovis Oncology, and GSK; participation on a data safety monitoring board or advisory board for Oncoinvest, Corcept, Sutro, AstraZeneca, ImmunoGen, GSK, Seagen, Genmab, Clovis Oncology, GSK, MSD, and PharmaMar; leadership or fiduciary role in other board, society, committee, or advocacy group, paid or unpaid, for GCIG; receipt of equipment, materials, drugs, medical writing, gifts, or other services from Clovis Oncology, GSK, MSD, and PharmaMar; and other financial or non-financial interests in AstraZeneca, Clovis Oncology, Genmab, GSK, ImmunoGen, Incyte, MSD, Roche, Seagen, and Novartis. YX reports participation on a data safety monitoring board or advisory board for MSD. KH reports research contracts from Daiichi Sankyo, Eisai, MSD, and Takeda; advisory board fees from Chugai, Eisai, Takeda, MSD, Roche, Genmab, and Sanofi; honoraria from Daiichi Sankyo, AstraZeneca, Chugai, Eisai, Genmab, MSD, Takeda, Sanofi, Kyowa Kirin, and Kaken; and support for attending meetings or travel from Regeneron. GS reports grants or contracts from MSD Italia; consulting fees from Tesaro and Johnson & Johnson; and payment or honoraria for lectures, presentations, speakers' bureaus, manuscript writing, or educational events from Clovis Oncology Italy. ML reports grants or contracts from MSD, AstraZeneca, and Roche; speaker fees from AstraZeneca and Roche; and meeting support from Roche. PR-E reports payment or honoraria for lectures, presentations, speakers' bureaus, manuscript writing, or educational events from MSD, Asofarma, Novartis, Janssen, and AstraZeneca; support for attending meetings or travel from Roche, Novartis, Asofarma, and AstraZeneca; and participation on a data safety monitoring board or advisory board for Pfizer, Asofarma, Bristol Myers Squibb, and AstraZeneca. NC reports participation on a data safety monitoring board or advisory board for GSK, Toray, Tarveda Therapeutics, Umoja, Kartos, Zentalis, Profound, and Novita Pharmaceuticals. AJPdSG reports consulting fees, payment or honoraria for lectures, presentations, speakers' bureaus, manuscript writing, or educational events, and participation on a data safety monitoring board or advisory board for AstraZeneca, Astellas, Bayer, and Janssen; payment for expert testimony from Janssen; and support for attending meetings or travel from Janssen, MSD, and Bayer. FCM reports speaker fees from MSD, Bristol Myers Squibb, GSK, Eli Lilly, and BMS; payment for expert testimony from MSD, Janssen, GSK, Eli Lilly, and Bristol Myers Squibb; and meeting and travel support from MSD and Amgen. AR reports advisory board fees from MSD Israel. J-YL reports grants or contracts from Advenchen, Ascendis Pharma, Alkermes, AstraZeneca, BeiGene, BerGenBio, Bristol Myers Squibb, Cellid, Clovis Oncology, Eisai, Genmab, GII, GSK, ImmunoGen, Janssen, Merck, Mersana, MSD, Novartis, Onconic Therapeutics, OncoQuest, Ono, Regeneron, Roche, Seagen, Sutro, Synthon, and Takeda; and payment or honoraria for lectures, presentations, speakers' bureaus, manuscript writing, or educational events from AstraZeneca, Janssen, MSD, Roche, Takeda, and Ono. FZ reports grants or contracts, consulting fees, payment for expert testimony, support for attending meetings or travel, participation on a data safety monitoring board or advisory board, and leadership or fiduciary role in other board, society, committee, or advocacy group, paid or unpaid, from AstraZeneca, Eli Lilly, MSD, Pfizer, Roche, Daiichi Sankyo, Merck, Novartis, Genesis Pharma, and Gilead. LG reports institutional grants from GSK, Pfizer, MSD, Karyopharm, Tesaro, IMV, Alkermes, Clovis Oncology, ImmunoGen, Roche, Mersana, and AstraZeneca; consulting fees from GSK, Merck, Eisai, and Novocure; payment or honoraria for lectures, presentations, speakers' bureaus, manuscript writing, or educational events from GSK, Merck, Eisai, Novocure, and Corcept; and support for attending meetings or travel from Corcept, GSK, AstraZeneca, and Merck. JS reports grants or contracts from Roche, MSD, GSK, Tesaro, AstraZeneca, Eisai, Merck, and Novocure; consulting fees from ImmunoGen, Incyte, GSK, AstraZeneca, Clovis Oncology, Novocure, MSD, Eisai, and Merck; payment or honoraria for lectures, presentations, speakers' bureaus, manuscript writing, or educational events from ImmunoGen, Incyte, GSK, AstraZeneca, Clovis Oncology, Novocure, Bristol Myers Squibb, Eisai, and Novartis; support for attending meetings or travel from GSK, AstraZeneca, Roche, Novocure, ImmunoGen, Incyte, MSD, and Eisei; participation on a data safety monitoring board or advisory board for ImmunoGen, Incyte, GSK, AstraZeneca, Clovis Oncology, Novocure, Bristol Myers Squibb, MSD, Merck, Bayer, and PharmaMar; and leadership or fiduciary role in other board, society, committee, or advocacy group, paid or unpaid, for ENGAGE, ESGO, ASCO, ESGO, GCIG, Deutsche Stiftung Eierstockkrebs, and AGO. KLin reports grants or contracts from GSK (paid to institution); payment or honoraria for lectures, presentations, speakers' bureaus, manuscript writing, or educational events from Eisai; and participation on a data safety monitoring board or advisory board for Eisai, MSD, Nykode, AstraZeneca, and GSK. HZ reports contracted research and payment or honoraria for lectures, presentations, speakers' bureaus, manuscript writing, or educational events for MSD. AO reports grants or contracts from AbbVie Deutschland, Advaxis, Aeterna Zentaris, Amgen, Aprea Therapeutics, Bristol Myers Squibb, Clovis Oncology, Eisai, Roche, ImmunoGen, MSD de España, Millennium Pharmaceuticals, PharmaMar, Regeneron, and Tesaro (paid to institution); consulting fees and participation on a data safety monitoring board or advisory board from Agenus, AstraZeneca, Clovis Oncology, Corcept, Deciphera Pharmaceuticals, Eisai, Exelixis, EMD Serono, Roche, Genmab, GSK, ImmunoGen, iTeos, MSD de España, Mersana, Novocure, OncXerna Therapeutics, PharmaMar, Regeneron, Sattucklabs, Seagen, and Sutro; payment or honoraria for lectures, presentations, speakers' bureaus, manuscript writing, or educational events from NSGO, Peerview, Peervoice, Medscape, Asociación Colombiana de Ginecológos Oncólogos, ESO, AstraZeneca, and GSK; and support for attending meetings or travel from AstraZeneca, PharmaMar, and Roche. SPo reports consulting fees from MSD, AstraZeneca, GSK, and Eisai; payment or honoraria for lectures, presentations, speakers' bureaus, manuscript writing, or educational events, support for attending meetings or travel, and advisory board fees from MSD, AstraZeneca, and GSK; and leadership or fiduciary role in other board, society, committee, or advocacy group for the Austrian Association of Gynecologic Oncology (unpaid). SPi reports research funding from Roche, MSD, AZ, Pfizer, and GSK; and honoraria from AZ, Roche, MSD, GSK, and PharmaMar. LRD reports research funding (paid to institution) from and acting as an expert advisor (unpaid) for Merck & Co; writing fees for expert content for UpToDate; participation on a scientific advisory board for Aadi Bioscience; and serving on the Editorial Board of the British Journal of Obstetrics and Gynaecology. KLi, KY, ST, and SMK are full-time employees of MSD and hold stock or restricted stock units in the company., (Copyright © 2024 Elsevier Ltd. All rights reserved.)