538 Background: Bladder cancer (BC) has a relatively high rate of human epidermal growth factor receptor 2 (HER2) alterations. The association between HER2 mutation/amplification, HER2, and PD-L1 immunohistochemistry (IHC) expression and associations with clinical outcomes for advanced bladder cancer (BC) has not been studied. Methods: We retrospectively analyzed BC samples for PD-L1 and HER2 IHC expression and compared HER2 alterations from genomic profiling with the MSK IMPACT platform. HER2 IHC expression was defined as 0, 1+, 2+, 3+, and PD-L1 IHC was the combined tumor and immune cell PD-L1 expression score (CPS). HER2 alteration was defined as either pathogenic mutation and/or amplification. We studied pairwise associations between HER2 alteration, PD-L1, and HER2 IHC expression in all patients and their associations with progression-free survival (PFS) and overall survival (OS) for muscle-invasive bladder cancer (MIBC) pts. Association analyses were performed using the Wilcoxon rank-sum test or Fisher’s exact test. Kaplan-Meier method and Cox proportional hazard models were used for time-to-event analyses. Results: Among 202 pts with HER2 IHC, 188 had MSK IMPACT, and 168 had PD-L1 CPS. The overall incidence of HER2 alteration was 22.3%; 48.2% had CPS ³10, and HER2 IHC distribution was 0:18.8%, 1+:29.7%, 2+:33.7%, and 3+:17.8%. The CPS score was inversely associated with HER2 IHC expression (p<0.001). No association was noted between CPS score and HER2 alteration (p=0.735). HER2 altered tumors were strongly correlated with high-level HER2 IHC expression (p<0.001). However, 41% (n=14/34) of HER2 IHC 3+ samples did not have HER2 alterations, and 17% (n=7/36) of HER2 altered samples had HER2 IHC expression of 0. In patients with MIBC, HER2 alteration and HER2 IHC expression (0/1+ vs. 2+/3+) were not associated with PFS (p=0.5 and p=0.4, respectively) or OS for MIBC pts (p=0.84 and p=0.94, respectively). A higher PD-L1 CPS score (>=10 vs <10) was associated with improved PFS for MIBC pts (p=0.03). Conclusions: Our study is the first to describe an inverse correlation between HER2 IHC expression and PD-L1 CPS score. Furthermore, HER2 IHC overexpression is strongly associated with HER2 amplification, but a subset of patients with high HER2 protein expression are potentially missed by genomic profiling alone. HER2 expression by IHC or HER2 genomic alteration is not a prognostic marker for MIBC pts in this cohort. This data provides the foundation for further HER2-directed advanced BC studies. [Table: see text]