9549Background: Antibodies against PD-1 have shown remarkable activity in the treatment of advanced melanoma. Clinical characteristics correlated with response have not yet been fully characterized. Previous work has shown that site of metastasis can correlate with treatment outcome. We sought to confirm these findings in a larger cohort of metastatic melanoma patients (pts). Methods: We conducted a multicenter retrospective study in a cohort of metastatic melanoma pts treated with anti-PD-1 agents. Relevant clinical data were retrieved from electronic medical records. Tumor responses were evaluated by RECIST v1.1. A multivariate regression model was used to investigate demographic and clinical variables associated with response. Results: We identified 337 pts who received pembrolizumab (2mg/kg or 10mg/kg Q3W) (N = 326) or nivolumab (3mg/kg Q2W) (N = 11) on clinical trials (N = 273) and early access programs (N = 64) at UCSF and USZ. In the multivariate model, pts who had liver metastasis (odds ratio [OR]...