Background: Obesity is associated with improved outcomes in melanoma patients (pts) treated with PD-1, whereas low muscle mass, known as sarcopenia, has been associated with poor outcomes in many cancers. We sought to assess the impact of body composition on PD-1 outcomes. Methods: We analyzed pre-treatment CT scans at the L3 slice using Slice-o-matic software (Tomovision V. 5.0) to determine skeletal muscle, visceral adipose, and subcutaneous adipose tissue parameters for 104 pts with metastatic melanoma who received PD-1 monotherapy. We assessed sarcopenia using skeletal muscle index (SMI=skeletal muscle area/m²). We also quantified total adipose tissue index (TATI), and skeletal muscle gauge (SMG = SMI x skeletal muscle density [SMD]). We stratified pts into high/low groups using previously published cutoffs and assessed toxicity (tox), progression-free and overall survival (PFS/OS), and response rate (RR) by group. Results: Sarcopenia (low SMI) was negatively associated with any tox (39% vs. 60%, p=0.04) but not OS, PFS, or RR. Adiposity (TATI) was not associated with outcomes. By contrast, SMG was significantly associated with OS (median 35.5 vs. 16.0m, p=0.01 for high vs. low SMG). Interestingly, when incorporating TATI with SMG, we found that high SMG/high TATI pts (high muscle/high fat) have superior clinical outcomes (Table). Notably, low SMG/high TATI pts (low muscle/high fat) had seemingly the worst outcomes. Conclusions: We found that high SMG, a measure incorporating muscle area and density, was associated with improved OS in PD1 treated pts. Further, pts with high adiposity and high SMG had superior outcomes, potentially identifying the population responsible for the favorable effect of obesity in these pts. Validation and combination treated cohorts will be presented.