BACKGROUND/AIM: To determine whether BMI and sarcopenia were related to treatment-limiting toxicity or efficacy of pembrolizumab treatment in melanoma patients. PATIENTS AND METHODS: Medical records for melanoma patients undergoing pembrolizumab treatment at Duke University from January 2014 to September 2018 were reviewed. Pre-treatment measurements such as BMI were collected. Pre-treatment CT imaging was used to determine psoas muscle index (PMI). Patients in the lowest sex-specific tertile of PMI were sarcopenic. Logistic regression measured associations with treatment toxicity and response. Kaplan-Meier analysis assessed progression-free survival (PFS) and overall survival (OS). RESULTS: Among 156 patients, the overall objective response rate was 46.2% and 29 patients (18.6%) experienced treatment-limiting toxicity. Sarcopenia was not significantly associated with toxicity, response, or survival. However, obese patients (BMI >30) experienced higher rates of toxicity (p=0.0007). CONCLUSION: Sarcopenia did not appear to predict clinically relevant outcomes. Obesity, however, represents a readily available predictor of pembrolizumab toxicity.