Background: Efficacy of PD-1 inhibitors (or any therapy) in NCC-RCC has not been established. KEYNOTE-427 (NCT02853344) is a single-arm, open-label, phase 2 study of pembro monotherapy in patients (pts) with advanced clear cell RCC (cohort A) and NCC-RCC (cohort B). Cohort B results are presented. Methods: 165 pts with histologically confirmed NCC-RCC, no prior systemic therapy, measurable disease (RECIST v1.1), and KPS ≥70% enrolled. Pts received pembro 200 mg IV Q3W for 35 cycles (~2 y) or until progressive disease (PD), unacceptable toxicity, or withdrawal. Primary end point: objective response rate (ORR) per RECIST v1.1 by blinded independent central review. Additional end points: duration of response (DOR), population description by sarcomatoid differentiation, histology and PD-L1 expression (combined positive score [CPS] ≥1 for PD-L1+). Results: Histology was confirmed by a central pathologist: papillary 72% (n = 118), chromophobe 13% (n = 21), unclassified 16% (n = 26); 62% were PD-L1+. At analysis, 49 pts had died and 3 had withdrawn. At median follow-up of 11.1 mo (range, 0.9-21.3), 56% of pts discontinued pembro due to PD or clinical progression. Overall ORR was 24.8% (95% CI, 18.5-32.2; 8 [4.8%] CR, 33 [20.0%] PR). Median DOR was not reached. For responding pts, 81.5% had a response ≥6 mo. 12-mo PFS and OS rates were 22.8% and 72.0%, respectively. ORR (95% CI) was 25.4% (17.9-34.3) with papillary, 9.5% (1.2-30.4) with chromophobe, and 34.6% (17.2-55.7) with unclassified NCC-RCC; for responding pts, 82.1%, 50.0%, and 87.5% had a response ≥6 mo, respectively. Median DOR was not reached in any group. ORR (95% CI) was 44.7% (28.6-61.7) for pts with sarcomatoid differentiation (n = 38). ORR (95% CI) was 33.3% (24.3-43.4) and 10.3% (3.9-21.2) with CPS≥1 and CPS < 1, respectively. Grade 3-5 treatment-related adverse events (TRAEs) occurred in 11% of pts. 6 pts died of AEs, 2 of TRAEs (pneumonia and cardiac arrest). Conclusions: Single-agent pembro showed encouraging antitumor activity in NCC-RCC, especially with papillary or unclassified histology. Safety profile of pembro was as expected. Updated data with additional follow-up will be presented. Clinical trial information: NCT02853344