Background: Obesity promotes PD-1–mediated T cell dysfunction but also improves tumor response to PD-1 blockade. Obesity has been linked with positive outcomes in pts treated with PD-1 blockade. To evaluate the prognostic utility of BMI, we performed a retrospective evaluation of BMI and other covariates in 172 pts with stage IV cutaneous MEL. Methods: Pts with stage IV cutaneous MEL who received anti PD-1 therapy at the University of Pittsburgh between 2014-2018 were included in this analysis. PD-1 blockade was continued until progression or intolerable toxicity. Tumor assessment was performed at baseline and every 12 weeks and response classified per RECIST v1.1. Clinical and demographic data were obtained. BMI was defined based on values at the first treatment date and dichotomized into two groups: ≥30 vs. < 30. Fisher exact test was used to evaluate the correlation between BMI group and ORR. Kaplan Meier method and Cox proportional hazard models were performed to analyzed the time-to-event outcomes (OS and PFS). Results: 172 pts with advanced MEL were evaluated. Greater BMI was associated with greater ORR, PFS and OS across various BMI cutoffs (BMI≥28, BMI≥30 and BMI≥35) although this effect was most obvious at BMI≥30. Pts with BMI≥30 achieved higher ORR than those with BMI < 30 (74% vs. 58%, p-value = 0.04). Concordantly, pts with BMI≥30 had improved PFS and OS: median PFS (BMI≥30 21.1 mos vs. BMI < 30 10.7 mos) and median OS (BMI≥30 35.4 mos vs. BMI < 30 22.8 mos). Higher BMI was independently associated with improved OS (p = 0.018) and PFS (p = 0.047) adjusting for age, Breslow thickness and sex. No significant interaction was observed between the effects of BMI and that of age, sex, or Breslow thickness. Conclusions: Increased BMI was associated with greater ORR in addition to previously reported associations with PFS/OS in a large retrospective series of advanced MEL pts treated with PD-1 blockade. This data was independent of other prognostic factors and underscore the “inflammaging” effects of obesity and age in relation to anti PD-1 therapy in advanced cancer.