Background: There is emerging data on the role of BMI and outcomes of ICIs in various solid tumors, with underweight pts having worse outcomes to ICIs. The effect of BMI on efficacy of ICIs in pts with mUC is not clear. Methods: We identified 335 adult pts with mUC at the Cleveland Clinic treated who received >/= 2 cycles of ICI with pembrolizumab (P) or atezolizumab (A) between 2015 and 2023. Patient characteristics including age, sex, race, primary site (bladder vs upper tract UC (UTUC)), tumor histology and pre-ICI treatment NLR values were collected. BMI was calculated and values were collected and compared to assess impact on overall (OS) and progression free survival (PFS) after ICI start date. BMI was broken down into four categories: underweight (BMI less than 18.5 kg/m2), normal weight (BMI between 18.5 and 24.9 kg/m2), obese (BMI 30 kg/m2 or greater) and overweight (BMI between 25 and 29.9 kg/m2). OS and PFS were estimated by Kaplan Meier method and compared using log rank test. Results: Median age of pts was 73 yrs (35-95) and 254 (76%) pts were males. 247 (74%) received P, 88 (26%) received A. We found that underweight pts had significantly worse OS compared to normal or higher BMI (2.32 vs 13.08 mos; p=0.001) as well a significantly worse PFS (1.61 vs 4.75 mos; p=0.02) (Table). Conclusions: In our large real-world cohort of pts with mUC receiving ICI, we report for the first time the effect of BMI on outcomes with ICI. Further prospective studies are warranted.