Background: A randomized phase III trial (JAVELIN Bladder 100; NCT02603432) to investigate avelumab (anti–PD-L1) as 1L maintenance therapy in patients with advanced UC met its primary objective, demonstrating significantly prolonged overall survival (OS) with Ave + BSC vs BSC alone in all randomized patients and in patients with PD-L1+ tumors. We report efficacy and safety in Japanese patients enrolled in this study. Methods: Eligible patients with unresectable locally advanced or metastatic UC that had not progressed with 4-6 cycles of gemcitabine with either cisplatin or carboplatin were randomized 1:1 to receive maintenance Ave (10 mg/kg IV every 2 weeks) + BSC or BSC alone, stratified by best response to 1L chemotherapy (complete/partial response vs stable disease) and by visceral vs nonvisceral disease when initiating 1L chemotherapy. The primary endpoint was OS, assessed from randomization in all randomized patients and in patients with PD-L1+ tumors (Ventana SP263 assay). Secondary endpoints included progression-free survival (PFS) per blinded independent central review and safety. Results: Japanese patients (n=73) were randomized to receive Ave + BSC (n=36) or BSC alone (n=37); 52.8% vs 62.2% had PD-L1+ tumors, respectively. Median OS (95% CI) was 24.7 months (18.2-not estimable [NE]) with Ave + BSC vs 18.7 months (12.8-33.0) with BSC alone (HR, 0.81 [95% CI; 0.409-1.585]) in all randomized patients and 18.6 months (9.4-NE) with Ave + BSC vs 19.4 months (11.7-33.0) with BSC alone (HR, 1.00 [95% CI, 0.413-2.412]) in patients with PD-L1+ tumors. Median PFS (95% CI) was 5.6 months (1.9-9.4) with Ave + BSC vs 1.9 months (1.9-3.8) with BSC alone (HR, 0.63 [95% CI, 0.358-1.113]) in all randomized patients and 5.6 months (1.8-11.2) with Ave + BSC vs 1.9 months (1.9-3.8) with BSC alone (HR, 0.62 [95% CI, 0.298-1.301]) in patients with PD-L1+ tumors. The most common treatment-emergent adverse events (all grade; grade ≥3) in the Ave + BSC arm were pyrexia (10 [27.8%]; 0), nasopharyngitis (7 [19.4%]; 0), and anemia (7 [19.4%]; 4 [11.1%]). Conclusions: Ave 1L maintenance + BSC was efficacious and tolerable in Japanese patients with advanced UC, and results were generally consistent with those in the overall population. Clinical trial information: NCT02603432