Background: Relapse after surgery for high-risk clear cell RCC (ccRCC) is associated with shortened life expectancy. Effective perioperative therapy to reduce this risk remains an unmet need. Adjuvant immune therapy is an attractive potential strategy for these pts. We conducted the KEYNOTE-564 trial to evaluate pembro vs placebo as adjuvant therapy for pts with RCC. Methods: KEYNOTE-564 is a phase III multicenter trial of pembro vs placebo in pts with histologically confirmed ccRCC, with intermediate-high risk (pT2, Gr 4 or sarcomatoid, N0 M0; or pT3, any Gr, N0 M0), high risk (pT4, any Gr, N0 M0; or pT any stage, any Gr, N+ M0), or M1 NED (no evidence of disease after primary tumor + soft tissue metastases completely resected ≤1 year from nephrectomy) (Leibovich et al, 2003; Fuhrman et al, 1982). Pts had undergone surgery ≤12 wks prior to randomization; had no prior systemic therapy; had ECOG PS 0 or 1. Study treatment was given for up to 17 cycles (≈1 yr). The primary endpoint was disease-free survival (DFS) per investigator assessment in all randomized pts (ITT population). Overall survival (OS) was a key secondary endpoint. Safety/tolerability were secondary endpoints, assessed in all treated pts. Results: Between Jun 30, 2017 and Sept 20, 2019, 994 pts were randomized 1:1 to pembro (n=496) or placebo (n=498). As of data cutoff date of Dec 14, 2020, median (range) follow-up, defined as time from randomization to data cutoff, was 24.1 (14.9−41.5) mo. No pts remain on study treatment. Baseline characteristics were generally balanced between arms. At first prespecified interim analysis, the primary endpoint of DFS was met (median not reached [NR] for both arms, HR 0.68, 95% CI 0.53−0.87; P=0.0010 [one-sided]). The estimated DFS rate at 24 mo was 77.3% with pembro vs 68.1% with placebo. Overall, DFS benefit was consistent across subgroups. A total of 51 OS events were observed (18 in the pembro arm, 33 in the placebo arm). Median OS was NR for both arms (HR 0.54, 95% CI 0.30−0.96; P=0.0164 [one-sided]); the p-value did not cross the statistical hypothesis testing boundary. The estimated OS rate at 24 mo was 96.6% with pembro vs 93.5% with placebo. 470 pts (96.3%) and 452 pts (91.1%) experienced ≥1 all-cause adverse events (AEs) with pembro vs placebo, respectively. Grade 3-5 all-cause AEs occurred in 158 pts (32.4%) with pembro and 88 pts (17.7%) with placebo. No deaths related to pembro occurred. Conclusions: Pembro demonstrated a statistically significant and clinically meaningful improvement in DFS vs placebo in pts with intermediate-high, high risk or M1 NED RCC. Additional follow-up is planned for the key secondary endpoint of OS. KEYNOTE-564 is the first positive phase III study with a checkpoint inhibitor in adjuvant RCC, and these results support pembro as a potential new standard of care for pts with RCC in the adjuvant setting. Clinical trial information: NCT03142334.