Introduction: In JAVELIN Bladder 100, avelumab first-line maintenance (1LM) improved overall survival (OS) and progression-free survival (PFS) in patients with locally advanced/metastatic urothelial carcinoma (la/mUC) without progression following 1L platinum-based chemotherapy (PBC) vs best supportive care. PATRIOT-II describes real-world outcomes with avelumab 1LM. Patients and Methods: This observational, retrospective study of avelumab 1LM in US community/academic centers used medical record data collected from avelumab initiation for ≥12 months to assess survival, safety, and healthcare resource utilization; analyses are descriptive. Results: The study included 160 patients from 37 centers (median age, 70 years; 77% male). Avelumab 1LM was initiated at a median of 4 weeks (IQR 3-6) after PBC completion. Median follow-up from avelumab 1LM was 16 months (IQR 11-21). At study end, 19.4% of patients continued avelumab; 73.7% had discontinued due to progression, adverse events (AEs), or performance status deterioration. Median PFS and OS from avelumab initiation were 5.4 months (95% CI 3.8-6.9) and 24.4 months (95% CI 20.4-28.4), respectively. Grade ≥3 treatment-related AEs (TRAEs) occurred in 15 patients (9.4%); 35 (21.9%) had any-grade immune-related AEs, and 23 (14.3%) received high-dose systemic corticosteroids for AEs. Forty-four patients (27.5%) were hospitalized during the avelumab treatment period, of whom 13 (8.1%) were hospitalized due to TRAEs. Limitations of this study include a small sample size, potential selection bias, and missing/unknown data. Conclusion: These results align with the JAVELIN Bladder 100 clinical trial and other real-world studies, supporting avelumab 1LM use in patients with la/mUC without progression following 1L PBC. MICRO-ABSTRACTIn this study, researchers studied 160 people with advanced urothelial cancer in the US receiving avelumab maintenance treatment. These people had already received platinum chemotherapy, and their disease had either gone away, become smaller, or stopped growing. The efficacy and safety of avelumab treatment in these people were comparable to findings from a previous clinical trial and other real-world studies.