Background: PD-1 blockade via pembro monotherapy showed antitumor activity in R/R cHL. KEYNOTE-204 (NCT02684292) was a randomized, international, open-label, phase III study of pembro vs BV in R/R cHL. Methods: Patients (pts) were aged ≥18 y, were post−autologous stem cell transplant (auto-SCT) or ineligible for auto-SCT, and had measurable disease and ECOG PS 0 or 1. BV-naive and BV-exposed pts were eligible. Pts were randomized 1:1 to pembro 200 mg IV Q3W or BV 1.8 mg/kg IV Q3W and stratified by prior auto-SCT (yes vs no) and status after 1L therapy (primary refractory vs relapsed <12 mo vs relapsed ≥12 mo after end of 1L therapy). Primary end points: PFS by blinded independent central review (BICR) per International Working Group (IWG) criteria including clinical and imaging data after auto-SCT or allogeneic SCT (allo-SCT) and OS. Key secondary end points: PFS excluding clinical and imaging data after auto-SCT or allo-SCT (PFS-secondary), and ORR by BICR per IWG, PFS by investigator review per IWG, and safety. Exploratory end point: DOR by BICR per IWG. Results: 304 pts were randomized and 300 were treated (148, pembro; 152, BV); 256 discontinued. Median (range) follow-up: 24.7 (0.6-42.3) mo. 15 pts were BV exposed. Median (range) time on treatment was 305.0 (1-814) and 146.5 (1-794) days with pembro and BV, respectively. Statistically significant improvement was observed with pembro vs BV for primary PFS analysis (HR 0.65 [95% CI 0.48-0.88; P =0.00271]; median 13.2 vs 8.3 mo); 12-mo PFS rates were 53.9% vs 35.6%, respectively. Benefit was observed in all subgroups tested, including pts with no auto-SCT (HR=0.61), primary refractory disease (HR=0.52), prior BV (HR=0.34) and BV naive (HR=0.67). Significant improvement in PFS-secondary was observed with pembro vs BV (HR 0.62 [95% CI 0.46-0.85]; median 12.6 vs 8.2 mo). Per investigator assessment, PFS was longer with pembro vs BV (HR 0.49 [95% CI 0.36-0.67]; median 19.2 vs 8.2 mo). ORR was 65.6% for pembro and 54.2% for BV; CR rates were 24.5% and 24.2%, respectively. Median (range) DOR was 20.7 mo (0.0+ to 33.2+) for pembro and 13.8 mo (0.0+ to 33.9+) for BV. Grade 3-5 TRAEs: 19.6% of pts with pembro and 25.0% with BV. One death due to TRAE occurred with pembro (pneumonia). Conclusions: In pts with R/R cHL, pembro was superior to BV and demonstrated statistically significant and clinically meaningful improvement in PFS across all subgroups, with safety consistent with previous reports. Pembro monotherapy should be standard of care for this pt population with R/R/cHL. Clinical trial information: NCT02684292.