Purpose: Emerging evidence has suggested that prior or concurrent antibiotic (ATB) use may be associated with a poor response to immune checkpoint inhibitors (ICIs) in patients with some solid tumors. This study examined the effects of ATB use on the oncological outcomes of patients receiving ICIs for mUC. Materials and Methods: Patients receiving ICIs for mUC between 2018 and 2020 were assessed retrospectively. Those with over three cycles of atezolizumab or pembrolizumab were included. ATB use, defined as ≥ three days within 60 days before or three months after ICI administration, was compared between groups for oncological outcomes. Results: Thirty-one patients were examined. The ATB-use and no-ATB-use groups consisted of 15 (48.4%) and 16 patients (51.6%), respectively. The ATB-use group showed a lower disease control rate (56.3% vs. 13.3%, p=0.023) than the no-ATB-use group. The objective response rate in the ATB-use group was lower than the no-ATB-use group, but the difference was statistically insignificant (43.7% vs. 13.3%, p=0.113). The ATB-use group had shorter progression-free survival (median three vs. six months, log-rank p=0.045) and shorter overall survival (median three vs. 14 months, log-rank p=0.023) than the no-ATB-use group. The most commonly used antibiotic...