Background: There is emerging evidence that patients (pts) treated with immune-checkpoint inhibitors (ICIs) may have a poorer response in the setting of antibiotic use (ABx), possibly due to negative impact on gut microbiota. Our group previously demonstrated that Abx use 60 days before or 60 days in mUC pts after initiation of ICI therapy did not have a significant impact on overall survival (OS) in real-world setting. We now studied the effect of Abx use within 30 days of initiation of ICI on OS in the same cohort of mUC patients. Methods: We performed a retrospective analysis of adult pts with mUC treated at the Cleveland Clinic between 2015 and 2020. Pts included in the study received at least 2 cycles of ICI therapy with either atezolizumab or pembrolizumab. Statistical analysis included study of OS in weeks using the Kaplan Meier method and rank log test, Fischer’s exact test, and Kruskal-Wallis test. Results: A total of 115 pts that received ICI therapy were included. 57 pts received atezolizumab and 58 pts received pembrolizumab. 38 pts (33%) received antibiotics and 77pts (67%) did not. The most commonly used Abx used were Cephalosporins (27%), Penicillins/Carbapenems (25%), Flouroquinolones (23%), and Bactrim (11%). 18 pts received Abx within 30 days before initiation of ICI, 13 pts received Abx within 30 days after initiation of ICI, and 7 pts received Abx before and after initiation of ICI. There was no statistical difference in OS in the group of pts that received Abx 30 days prior to initiation of ICI with median OS of 5.95 months (95% CI 3.22-13.67, p=0.0695) compared to 12.39 months (95% CI 10.09 – 18.6) in those who did not receive Abx. Similarly, there was no statistical difference in OS in the group of pts that received Abx 30 days after initiation of ICI with median OS of 5.09 months (95% CI 2.53-22.57, p=0.2339) compared to 12.02 months (95% CI 8.6`-17.02 Table). Conclusions: In our single institution study of mUC patients receiving ICI treatment, the use of Abx did not affect the OS. Although there was a trend for better OS seen in pts who did not receive Abx, it was not statistically significant (Table). Due to the limitations of a retrospective analysis and small sample size, further studies are warranted taking into account other factors that may affect gut microbiota in mUC pts.