Immune checkpoint inhibitors (ICIs) have transformed the treatment paradigm in muscle-invasive bladder cancer (MIBC). However, not all patients have consistent responses to these agents, and predictive biomarkers are still needed. Antibiotic (AB) use can potentially decrease the efficacy of ICIs in advanced stage MIBC by altering the microbiome and the antitumor immune response. This detrimental effect of AB use was not evaluated in the neoadjuvant setting.