Background: In the JAVELIN Bladder 100 phase 3 trial, avelumab first-line maintenance (AVE 1LM) + best supportive care (BSC) improved overall survival (OS) and progression-free survival (PFS) vs BSC alone in patients (pts) with locally advanced/metastatic urothelial carcinoma (la/mUC) with no disease progression following 4-6 cycles of platinum-based chemotherapy (PBC). PATRIOT-II aimed to describe real-world outcomes in pts with la/mUC treated with AVE 1LM who were progression-free after 1L PBC. We hypothesized that outcomes would be similar to those of JAVELIN Bladder 100. Methods: PATRIOT-II is an observational, retrospective study in US pts with la/mUC treated in geographically dispersed community and academic centers. Pts receiving AVE 1LM were eligible. Data were collected via medical records for a minimum of 52 weeks (wks) from AVE 1LM initiation. This analysis focuses on ≥24 wks post AVE 1LM initiation. Survival and safety outcomes were assessed; analyses are descriptive in nature. Results: 160 pts from 37 sites, a mix of oncology practices and community/academic centers, were included with median age, 70 (range, 40-90) years; 123 (77%) male; 118 (74%) White, non-Hispanic; 77 (49%) lower tract and 49 (31%) upper tract tumors; 119 (74%) ECOG PS 0/1; 70 (44%) and 51 (32%) known visceral (excluding bone) or non-visceral metastases (mets), respectively; 31 (19%) liver mets; and 64 (40%) creatinine clearance >60 mL/min. 100 (62.5%) and 60 (37.5%) pts had received 1L cisplatin or carboplatin, respectively. 130 pts (objective response rate, 81% [complete response, 21; partial response, 109]) had an investigator-assessed response to 1L PBC. Pts initiated AVE 1LM at a median of 4 wks (IQR, 3-6) after PBC completion. At data cutoff, 120 pts (75%) had discontinued AVE 1LM, 89 (74% of 120) due to progression/adverse events (AEs)/other non-death related reasons. Median time on AVE 1LM was 4.1 months (mo) (IQR, 2.3-8.7); median follow-up time post AVE 1LM initiation was 11.5 mo (IQR, 7.5-16.3). Table shows OS and PFS data. Most common documented treatment-related AEs (any grade) included hypothyroidism (n=7 [4%]), anemia (n=6 [4%]), blood creatinine increased (n=6 [4%]), fatigue (n=6 [4%]), and nausea (n=6 [4%]). Grade 3+ AEs occurred in 19 (12%), with no Grade 5 AEs. No new safety signals were noted. Conclusions: Results complement JAVELIN Bladder 100 and align with other AVE 1LM real-world studies. Limitations include retrospective data, lack of randomization and central scan review, missing data, selection bias, and confounders. PATRIOT-II further supports the level I evidence for AVE 1LM as standard of care in la/mUC not progressing on 1L PBC.