9545 Background: We previously showed that standard-dose pembro plus reduced-dose ipi has manageable safety and robust antitumor activity in patients (pts) with advanced melanoma. Here, we present more mature data, including 1-y landmark PFS and OS estimates. Methods: In the phase 1 KEYNOTE-029 expansion cohort (NCT02089685), pts with advanced melanoma, ECOG PS 0-1, no active brain metastases, and no prior immune checkpoint inhibitor therapy received pembro 2 mg/kg Q3W + ipi 1 mg/kg Q3W for 4 doses, then pembro alone for up to 2 y. Primary end point was safety. Efficacy end points were ORR, PFS, and DOR per RECIST v1.1 by independent central review and OS. Results: 153 pts were enrolled between Jan 13, 2015, and Sep 17, 2015. Median age was 60 y, 66% were male, 25% had elevated LDH, 56% had stage M1c disease, 36% were BRAF^V600mutant, and 13% received ≥1 prior therapy. As of Oct 17, 2016, median follow-up was 17 mo, and 64 (42%) pts remained on pembro. 110 (72%) pts received all 4 ipi doses. There were no treatment-related (TR) deaths. TRAEs occurred in all pts, were grade 3/4 in 69 (45%), and led to discontinuation of pembro and ipi in 17 (11%), ipi alone in 11 (7%), and pembro alone after ipi completion or discontinuation in 19 (12%). PD occurred in 1/11 pts who discontinued ipi alone and 4/17 pts who discontinued ipi and pembro. Of the 11 pts who discontinued ipi alone for a TRAE, 0 experienced recurrence of the same TRAE during pembro monotherapy and 2 discontinued pembro for a different TRAE (both elevated lipase). Immune-mediated AEs occurred in 90 (59%) pts and were grade 3/4 in 39 (25%). With 7 mo additional follow-up, there were 6 additional responses for an ORR of 61% (95% CI, 53%-69%); the CR rate increased from 10% to 15%. Median DOR was not reached (range, 1.6+ to 18.1+ mo), with 86/93 responders (92%), including 23/23 (100%) with CR, alive and without subsequent PD at cutoff. Median PFS and OS were not reached; 1-y estimates were 69% for PFS and 89% for OS. Conclusions: Pembro 2 mg/kg plus 4 doses of ipi 1 mg/kg has a manageable toxicity profile and provides robust, durable antitumor activity in pts with advanced melanoma. Clinical trial information: NCT02089685.