A randomized phase II trial confirmed the efficacy and acceptable tolerability of chemotherapy combined with the poly(ADP-ribose) polymerase inhibitor olaparib for the treatment of platinum-sensitive, recurrent, high-grade serous ovarian cancer. Median progression-free survival in 81 patients treated with olaparib, paclitaxel and carboplatin, followed by olaparib alone, was 12.2 months, compared with 9.6 months in 81 patients receiving initial chemotherapy, then no treatment. The survival benefit of olaparib treatment was greatest for patients with BRCA1 or BRCA2 mutations.