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Background: Docetaxelhasimmunostimulatoryeffectsthatmaypromoteanimmunor-esponsive prostate tumour microenvironment, providing a rationale for combination with nivolu-mab (programmed death-1 inhibitor) for metastatic castration-resistant prostate cancer (mCRPC). Methods: In the non-randomised, multicohort, global phase II CheckMate 9KD trial, 84 patients with chemotherapy-naive mCRPC, ongoing androgen deprivation therapy and (cid:1) 2 prior novel hormonal therapies (NHTs) received nivolumab 360 mg and docetaxel 75 mg/m 2 every 3 weeks with prednisone 5 mg twice daily ( (cid:1) 10 cycles) and then nivolumab 480 mg every 4 weeks ( (cid:1) 2 years). The co-primary end-points were objective response rate (ORR) and prostate-specific antigen response rate (PSA 50 -RR; (cid:3) 50% decrease from baseline). Results: The confirmed ORR (95% confidence interval [CI]) was 40.0% (25.7 e 55.7), and the confirmed PSA 50 -RR (95% CI) was 46.9% (35.7 e 58.3). The median (95% CI) radiographic progression-free survival (rPFS) and overall survival (OS) were 9.0 (8.0 e 11.6) and 18.2 (14.6 e 20.7) months, respectively. In subpopulations with versus without prior NHT, the ORR was 38.7% versus 42.9%, the PSA 50 -RR was 39.6% versus 60.7%, the median rPFS was 8.5 versus 12.0 months and the median OS was 16.2months versus not reached. Homologous recombination deficiency status or tumour mutational burden did not appear to impact efficacy. The most common any-grade and grade 3 e 4 treatment-related adverse events were fatigue (39.3%) and neutropenia (16.7%), respectively. Three treatment-related deaths occurred (1 pneumonitis related to nivolumab; 2 pneumonias related to docetaxel). Conclusions: Nivolumab plus docetaxel has clinical activity in patients with chemotherapy-naı¨ve mCRPC. Safety was consistent with the individual components. These results support further investigation in the ongoing phase III CheckMate 7DX trial. ClinicalTrials.