Background: The biomarkers PD-L1, FOXP3, and CD8 have been explored in pts with advanced UC who progressed after platinum-based chemotherapy (CTx). However, their relevance earlier in the disease process is less well understood. Methods: The Phase 2/3 LaMB study (NCT00949455) compared maintenance lapatinib vs placebo after first-line (1L) platinum-based CTx in pts with HER1/HER2-overexpressing stage IV advanced UC. Pre-CTx archival samples from this study were retrospectively analyzed and included both randomized and screen failure pts. PD-L1 expression was assessed (VENTANA SP263 Assay) and categorized as high (≥25% of tumor cells [TC] and/or immune cells [IC]) or low/negative ( < 25% TC and IC). Overall survival (OS) and progression-free survival (PFS) were estimated via Kaplan-Meier method; results were stratified by PD-L1 expression. The exploratory biomarkers CD8 and FOXP3 were also analyzed. The prognostic significance of the biomarkers was explored by multivariable Cox proportional hazards models and a bootstrap method for model selection. Results: Of 446 pts (232 randomized; 214 screened), 243 (54.5%) were assessed for PD-L1 expression, with 61 (25.1%) PD-L1 high and 158 (65.0%) PD-L1 low/negative. In PD-L1 high and low/negative pts, respectively, median OS (95% CI) was 12.0 (9.4–19.7) vs 12.5 months (10.4–15.5); median PFS (95% CI) was 6.5 (3.5–8.8) vs 5.0 months (4.3–6.3). PD-L1 expression was not associated with OS or PFS in univariate analysis or in a multivariate model for OS (hazard ratio [HR] for PD-L1 high vs low/negative 1.4 [95% CI, 0.8–2.3]). In a multivariate model for PFS, PD-L1 expression improved accuracy of the model by 23% and was a significant variable (HR, 2.1 [95% CI, 1.2–3.5]). Results of analyses of CD8 and FOXP3 will also be reported. Conclusions: Overall, these data suggest a lack of association between PD-L1 expression and survival in pts receiving 1L platinum-based CTx. Mechanisms underlying the potential association of PD-L1 expression with PFS remain unclear. CD8 and FoxP3 exploratory analyses may help to elucidate these results. Clinical trial information: NCT00949455