Background: Previously presented data from cohorts (C) 1-3 of the phase 2 KEYNOTE-199 study (n = 258; NCT02787005) showed that pembrolizumab monotherapy had antitumor activity and acceptable safety in patients (pts) with mCRPC previously treated with next-generation hormonal agents (NHAs; eg, abiraterone, enzalutamide) and docetaxel. Activity was observed for both PD-L1–positive and negative cohorts and in RECIST-measurable and bone-predominant disease. We present data from KEYNOTE-199 C1, C2, and C3 based on longer-follow-up. Methods: C1 enrolled 133 pts with RECIST-measurable, PD-L1–positive disease, C2 enrolled 66 pts with RECIST-measurable, PD-L1–negative disease, and C3 enrolled 59 pts with nonmeasurable, bone-predominant disease. All pts had ECOG PS 0-2 and received ≥1 NHA and 1-2 prior chemotherapies including docetaxel. Pembrolizumab 200 mg Q3W was given for 35 cycles or until PD or intolerable toxicity. Response was assessed Q9W in yr 1, then Q12W. Primary end point was ORR per RECIST v1.1 by central review. Key secondary end points included DCR (CR + PR + SD ≥6 mo), duration of response (DOR), OS, and safety. Results: Median follow-up as of Aug 21, 2018, was 9.5 mo in C1, 7.9 mo in C2, and 14.1 mo in C3. ORR (95% CI) was 5% (2-11) in C1 and 3% ( < 1-11) in C2. DCR was 10% in C1, 9% in C2, and 22% in C3 per RECIST v1.1. Median (range) DOR was not reached (1.9-21.8+ mo) in C1 and 10.6 mo (4.4-16.8) in C2; KM estimates of DOR ≥12 mo were 71% and 50%. Median (95% CI) OS was 9.5 mo (6.4-11.9) in C1, 7.9 mo (5.9-10.2) in C2, and 14.1 (10.8-17.6) in C3. 12-mo OS rates were 41% in C1, 35% in C2, and 62% in C3; 18-mo rates were 30%, 21%, and 36%. Grade 3-5 drug-related AE rates were 15% in C1, 14% in C2, and 17% in C3. There were 2 drug-related deaths (n = 1 each sepsis and pneumonitis). Conclusions: Pembrolizumab shows antitumor activity and disease control with acceptable safety in RECIST-measurable and bone-predominant mCRPC previously treated with NHAs and docetaxel. Responses are durable, and the observed OS benefit is promising. Clinical trial information: NCT02787005