4568 Background: Urothelial bladder carcinomas (UBC) with genomic alterations (GA) in the Fibroblast Growth Factor Receptor ( FGFR) genes have been postulated to be less responsive to immune checkpoint inhibitors (ICI). Immune microenvironment of these tumors could be altered due to suppression of interferon signaling pathways. Here, we present comprehensive genomic profiling (CGP) of FGFR altered UBC to study the underlying immunogenomic mechanisms of response and resistance. Methods: 4,035 UBC underwent hybrid capture based CGP. Tumor mutational burden (TMB) was determined on up to 1.1 Mbp of sequenced DNA and microsatellite instability (MSI) was determined using 114 loci. Programmed death ligand (PD-L1) expression in tumor cells was assessed by IHC (Dako 22C3). Results: 894 (22%) of UBC featured FGFR GA ( FGFR1 3.7%; FGFR2 1.1%; FGFR3 17.4%). Gender and age distribution was similar in all groups. FGFR3 cases had lower GA/tumor and 14.7% GA were fusions. ERBB2 amplification was significantly higher in FGFR1/2 altered UBC compared with FGFR3 altered UBC. MTOR pathway GA were highest in FGFR3 altered UBC. FGFR3 altered UBC featured significantly higher frequencies of biomarkers predicting resistance to ICI including lower TMB, lower PD-L1 expression and higher frequencies of GA in MDM2. FGFR3 driven UBC also features significantly higher frequencies of CDKN2A/B loss and MTAP loss which have also recently been linked to IO drug resistance. Conclusions: UBC harboring FGFR GA have increased frequency of alterations that have been linked to ICI resistance. Further evaluation of FGFR-based biomarkers in UBC clinical trials focused on the assessment of the patient response to ICI appears warranted. [Table: see text]