Background: The results of a phase I study in MEL suggested complementary clinical activity with NIVO (a PD-1 checkpoint inhibitor) plus IPI (a CTLA-4 checkpoint inhibitor). Here, we report the results of a randomized, double-blind, phase III trial designed to evaluate NIVO combined with IPI or NIVO alone vs IPI alone in MEL. Methods: Treatment-naïve pts (N = 945) were randomized 1:1:1 to NIVO 1 mg/kg Q2W + IPI 3 mg/kg Q3W for 4 doses followed by NIVO 3 mg/kg Q2W, NIVO 3 mg/kg Q2W + placebo, or IPI 3 mg/kg Q3W for 4 doses + placebo, until progression or unacceptable toxicity. Pts were stratified by PD-L1 status, BRAF mutation status, and M-stage. Co-primary endpoints are progression-free survival (PFS) (reported here) and overall survival (pts continue to be followed). Secondary endpoints include objective response rate (ORR) by RECIST v1.1 and safety. Results: At a minimum follow-up of 9 months, NIVO + IPI and NIVO alone significantly improved PFS and ORR vs IPI (Table). Grade 3-4 drug-related adverse events (AEs) occurred in 55.0%, 16.3%, and 27.3% of pts in the NIVO + IPI, NIVO, and IPI arms, respectively (most commonly diarrhea [9.3%, 2.2%, 6.1%], increased lipase [8.6%, 3.5%, 3.9%], increased alanine aminotransferase [8.3%, 1.3%, 1.6%], and colitis [7.7%, 0.6%, 8.7%]). Drug-related AEs led to discontinuation in 36.4%, 7.7%, and 14.8% of pts in the NIVO + IPI, NIVO, and IPI arms, with 0, 1, and 1 drug-related deaths, respectively. Efficacy outcomes by PD-L1 status will also be presented. Conclusions: NIVO + IPI and NIVO alone had superior clinical activity vs IPI alone. The results with NIVO + IPI and NIVO alone further suggest complementary activity of the two agents. There were no new safety signals or drug-related deaths observed with the combination. Clinical trial information: NCT01844505

*P < 0.00001 vs IPI. **Study not statistically powered for this comparison.