Background: There is a high unmet need for treatment regimens that increase the chance of long-term remission and possibly cure for women with newly diagnosed advanced ovarian cancer. In the primary analysis of SOLO1/GOG 3004, the poly(ADP-ribose) polymerase (PARP) inhibitor olaparib significantly improved progression-free survival versus placebo in patients with a BRCA mutation; median progression-free survival was not reached. Here, we report an updated, post-hoc analysis of progression-free survival from SOLO1, after 5 years of follow-up., Methods: SOLO1 was a randomised, double-blind, placebo-controlled, phase 3 trial, done across 118 centres in 15 countries, that enrolled patients aged 18 years or older with an Eastern Cooperative Oncology Group performance status of 0-1 and with BRCA-mutated, newly diagnosed, advanced, high-grade serous or endometrioid ovarian cancer with a complete or partial clinical response after platinum-based chemotherapy. Patients were randomly assigned (2:1) via a web-based or interactive voice-response system to receive olaparib (300 mg twice daily) or placebo tablets orally as maintenance monotherapy for up to 2 years; randomisation was by blocks and was stratified according to clinical response after platinum-based chemotherapy. Patients, treatment providers, and data assessors were masked to group assignment. The primary endpoint was investigator-assessed progression-free survival. Efficacy is reported in the intention-to-treat population and safety in patients who received at least one dose of treatment. The data cutoff for this updated, post-hoc analysis was March 5, 2020. This trial is registered with ClinicalTrials.gov (NCT01844986) and is ongoing but closed to new participants., Findings: Between Sept 3, 2013, and March 6, 2015, 260 patients were randomly assigned to olaparib and 131 to placebo. The median treatment duration was 24·6 months (IQR 11·2-24·9) in the olaparib group and 13·9 months (8·0-24·8) in the placebo group; median follow-up was 4·8 years (2·8-5·3) in the olaparib group and 5·0 years (2·6-5·3) in the placebo group. In this post-hoc analysis, median progression-free survival was 56·0 months (95% CI 41·9-not reached) with olaparib versus 13·8 months (11·1-18·2) with placebo (hazard ratio 0·33 [95% CI 0·25-0·43]). The most common grade 3-4 adverse events were anaemia (57 [22%] of 260 patients receiving olaparib vs two [2%] of 130 receiving placebo) and neutropenia (22 [8%] vs six [5%]), and serious adverse events occurred in 55 (21%) of 260 patients in the olaparib group and 17 (13%) of 130 in the placebo group. No treatment-related adverse events that occurred during study treatment or up to 30 days after discontinuation were reported as leading to death. No additional cases of myelodysplastic syndrome or acute myeloid leukaemia were reported since the primary data cutoff, including after the 30-day safety follow-up period., Interpretation: For patients with newly diagnosed advanced ovarian cancer and a BRCA mutation, after, to our knowledge, the longest follow-up for any randomised controlled trial of a PARP inhibitor in this setting, the benefit derived from 2 years' maintenance therapy with olaparib was sustained beyond the end of treatment, extending median progression-free survival past 4·5 years. These results support the use of maintenance olaparib as a standard of care in this setting., Funding: AstraZeneca; Merck Sharpe & Dohme, a subsidiary of Merck & Co, Kenilworth, NJ, USA., Competing Interests: Declaration of interests SB reports clinical trial support paid to her institution from AstraZeneca for this study; grants to her institution from AstraZeneca, GlaxoSmithKline and Tesaro; personal consulting fees from Amgen, AstraZeneca, Genmab, GlaxoSmithKline, Immunogen, Mersana, Merck Sharpe & Dohme, Merck Serono, Oncxerna, Pfizer, and Roche; personal fees from Amgen, AstraZeneca, Clovis Oncology, GlaxoSmithKline, Pfizer, Takeda, and Tesaro; support for attending a meeting or travel from Nucana; unpaid participation on a Epsilogen advisory board; and an unpaid role as director of membership for the European Society of Medical Oncology. KNM reports clinical trial support paid to her institution for this study; contracts from Genentech/Roche, Lilly Pharmaceuticals, and PTC Therapeutics for ovarian cancer investigator-initiated trials; consulting fees from IMab; payment to her institution for educational content in gynaecological cancers from Onc Live, Physician Education Resource (PER), PRIME Oncology, and Research to Practice; payment to her institution for advisory boards for use of assets in gynaecological cancers from Alkemeres, Aravive, Blueprint Pharmaceuticals, Eisai, Genentech/Roche, Immunogen, Mersana, Mereo, and VBL Therapeutics; participation on a data safety monitoring board for Incyte; and payments to her institution for being an associate director of GOG partners and committee chair for NRG Ovarian Cancer. NC reports grants from AstraZeneca, PharmaMar, and Roche; personal consulting fees from AstraZeneca, BIOCAD, Clovis Oncology, Eisai, GlaxoSmithKline, Immunogen, Merck Sharp & Dohme, Mersana, Oncxerna, Pfizer, PharmaMar, Roche, and Tesaro; and personal fees from AstraZeneca, Clovis Oncology, Eisai, GlaxoSmithKline, Merck Sharp & Dohme, Novartis, and Tesaro. GS reports grants and research support from Merck Sharpe & Dohme Italia; consulting fees from Johnson & Johnson and Tesaro Bio Italy; and speakers bureau fees and honoraria from Clovis Oncology Italy. AOa reports grants paid to her institution from AbbVie Deutschland, Abililty Pharmaceuticals, Advaxis, Aeterna Zentaris, Amgen, Aprea Therapeutics, Bristol Myers Squibb, Clovis Oncology, Eisai, F Hoffmann-La Roche, Immunogen, Merck Sharp & Dohme de España, Millennium Pharmaceuticals, Pharmamar, Regeneron Pharmaceuticals, and Tesaro; personal fees from AstraZeneca, Clovis Oncology, Deciphera Pharmarceutia, Genmab, GlaxoSmithKline, Immunogen, Mersana Therapeutic, PharmaMar, Roche, Sutro, and Tesaro; and support for attending meetings or travel, or both from AstraZeneca, Pharmamar, and Roche. MF reports personal advisory board and lecture fees, support to travel to a meeting and a grant to his institution from AstraZeneca; personal advisory board fees and a grant to his institution from Novartis; personal advisory board fees from GlaxoSmithKline, Lilly, Merck Sharpe & Dohme, and Takeda; personal lecture fees from Act Genomics and GlaxoSmithKline; research support to his institution from BeiGene; consulting for AbbVie (not renumerated); and participation on the Australasian Gastro-Intestinal Trials Group Independent Data Monitoring and Safety Committee. AF reports support for attending a medical congress from AstraZeneca. ALe reports grants from AstraZeneca and Sanofi; consulting fees from Seattle Genetics; honoraria or reimbursement and advisory board fees from AstraZeneca; advisory board fees or continuing medical education from Ability Pharma, Biocad, Clovis Oncology, GlaxoSmithKline, Medscape, Merck Serono, Merck Sharpe & Dohme, TouchCongress, and Zentalis; and support for attending meetings or travel, or both, from AstraZeneca, Clovis Oncology, GlaxoSmithKline, and Roche; and participation on a data safety monitoring board or advisory board for ARIEL4 and TROPHIMMUNE. GSS reports institutional research support from AstraZeneca and Merck for this study; institutional research support from Novartis and Roche; and consulting fees paid to his institution from Biovica and Seagen. CG reports clinical trial funding for this study to his institution from AstraZeneca; clinical research grants to his institution from Aprea, AstraZeneca, BergenBio, Clovis, GlaxoSmithKline, Medannexin, Merck Sharpe & Dohme, Novartis, Nucana, and Tesaro; personal consulting fees from AstraZeneca, GlaxoSmithKline, Merck Sharpe & Dohme, and Tesaro; honoraria for lectures or presentations from AstraZeneca, Chugai, Clovis Oncology, GlaxoSmithKline, Merck Sharpe & Dohme, Nucana, Roche, Takeda, and Tesaro; honoraria for lectures, presentations, or preparing educational materials from Cor2Ed; advisory board attendance for AstraZeneca, Chugai, GlaxoSmithKline, Merck Sharpe & Dohme, Nucana, Roche, and Tesaro; and being a committee member on the Scottish Medicines Consortium. AOz reports a grant from AstraZeneca to his institution outside the submitted work. AG-M reports clinical trial funding from GlaxoSmithKline and Roche; consulting fees from Alkermes, Amgen, AstraZeneca, Clovis, Genmab, GlaxoSmithKline, Immunogen, Mersana, Merck Sharpe & Dohme, Oncoinvent, Pharmamar, Roche, Sotio, and Takeda; personal fees from AstraZeneca, Clovis, GlaxoSmithKline, Merck Sharpe & Dohme, and Roche; support for attending meetings or travel, or both, from AstraZeneca, GlaxoSmithKline, Merck Sharpe & Dohme, Pharmamar, and Roche; and being the current chairman of GEICO and the chairman of ENGOT from 2018 to 2020. CA reports receiving advisory board fees from AbbVie, AstraZeneca/Merck, Eisai/Merck, Mersana Therapeutics, Repare Therapeutics, and Roche/Genentech; participation on an advisory board for Blueprint Medicine; participation on the board of directors for GOG Foundation and NRG Oncology; clinical trial funding to her institution from AstraZeneca for this study; and clinical trial funding to her institution from AbbVie, AstraZeneca, Clovis, and Genentech. ESL reports full-time employment with AstraZeneca during the conduct of the study and AstraZeneca stock ownership. EH reports full-time employment with AstraZeneca, contracted by PHASTAR, during the conduct of the study. B-GK, ALi, WHB, and PDS declare no competing interests., (Copyright © 2021 Elsevier Ltd. All rights reserved.)