INTRODUCTION: Immunotherapies have increased the therapeutic options for patients with metastatic renal cell carcinoma (mRCC), but the absence of prognostic indicators remains unresolved. We assessed the potential association of BMI with the overall survival (OS) of patients treated with nivolumab. METHODS: We performed a retrospective study of 126 patients with histologically confirmed mRCC who began systemic ICI therapy between January 2016 and April 2022 were included. BMI at the time of treatment start was calculated. Then patients were divided into two groups: high BMI (≥25) and low BMI (<
25). Therapeutic responses were determined according to RECIST v1.1. OS was defined as the time from starting ICI treatment until death or last follow-up at the data cutoff. RESULTS: The cohort was male 74.6%, with a median age of 62. The median follow-up time was 18.6 months. The patients were classified as low BMI (<
25) and high BMI (≥25). The OS was 40.6 months (95% CI: 34.2-47.0) for patients with high BMI vs. 9.4 months (95% CI: 7.0-11.7) for patients with low BMI, and a significant association was found between BMI and OS ( CONCLUSIONS: BMI was an independent prognostic factor in the patients with mRCC treated with nivolumab. Prospective and multicenter research is needed to confirm our findings.