Background: Elevated inflammatory biomarkers like the neutrophil to lymphocyte ratio (NLR) has been associated with poor prognosis in several cancers, including UC but there is limited evidence on its role as a prognostic biomarker in patients (pts) with mUC treated with ICIs. We studied the effect of baseline (pre-treatment) NLR on outcomes with ICI in mUC pts in our large cohort of patients with mUC treated with ICIs. 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 and divided into four quartiles: (NLR <2.7, 2.7-4.0, 4.0-7.1, and >7.1). Impact of NLR on overall survival (OS) and progression free survival (PFS) post ICI start date was studied. OS and PFS were estimated using the Kaplan Meier method and compared by log rank test. Results: Of the 335 pts, NLR values were available for 320 pts. Median age was 73 yrs (35-95) and 76% pts were males. 247 pts (74%) received P and 88 (26%) received A. We found that in the 320 patients with available NLR values, the highest quartile values of NLR >/= 7.1 was significantly associated with worse OS (p=0.01). We did not find a statistically significant impact of NLR on PFS (p=0.06). (Table). Conclusions: In our large real-world cohort of pts with mUC receiving ICI, we report the effect of baseline NLR on outcomes with ICI and that NLR >/=7.1 was associated with significantly worse OS. Further validation studies are warranted to risk-stratify pts with mUC planned for ICI treatment.