Background: KEYNOTE-427 (NCT02853344), a single-arm, open-label, phase 2 study, showed antitumor activity with first-line pembro monotherapy in nccRCC (cohort B). Studies of RCC and immune-oncology have shown that depth of tumor response may correlate with long-term benefit. We present the association between depth of response and OS plus updated efficacy and safety data in cohort B. Methods: Pts with histologically confirmed nccRCC, who did not receive prior systemic therapy, and who have measurable disease (RECIST v1.1) received pembro 200 mg IV Q3W for 2 y or until progressive disease, unacceptable toxicity, or withdrawal. End points were ORR (primary), DOR, and PFS (RECIST v1.1); OS; and safety. Association between depth of response, defined as maximum reduction from baseline in sum of target lesions, and OS was evaluated using a Cox proportional hazards model with target lesion reduction group as time-varying covariate. Results: Of 165 pts, 72% had papillary histology, 13% had chromophobe histology, and 16% were unclassified. Median time from enrollment to data cutoff was 18.7 mo (range, 9.9-26.0). ORR was 26.1% (95% CI, 19.5-33.5; 10 CRs, 33 PRs). Median (range) DOR was 15.3 mo (2.8-21.0+); 57.3% had DOR ≥12 mo. At 18-mo, PFS rate was 18.9% and OS rate was 67.0%. Most pts (58.8%) had some reduction in target lesions. Pts with a > 30% reduction in target lesions had an increased probability of survival (Table). ORR (95% CI) was similar for papillary (28.0% [20.1-37.0]) and unclassified (30.8% [14.3-51.8]) histology but lower for chromophobe (9.5% [1.2-30.4]). OS rates at 18 mo were 70.8%, 66.7%, and 50.0 in the papillary, chromophobe, and unclassified groups, respectively. Treatment-related AEs (TRAEs) occurred in 67.9% of all pts, primarily pruritus (19%), hypothyroidism (14%), and fatigue (14%). Grade 3-5 TRAEs occurred in 14% of pts; 2 pts died of TRAEs (pneumonia and cardiac arrest). Conclusions: First-line pembro monotherapy continued to show antitumor activity in nccRCC with no new safety concerns. In general, for pts who had greater reductions in target lesions, the trend was toward improved OS; pts with reduction of tumor burden ≥80% had comparable long term outcomes to those who achieved a RECIST 1.1 defined CR. Clinical trial information: NCT02853344.