Background: In the JAVELIN Bladder 100 trial, avelumab 1LM + best supportive care (BSC) significantly prolonged overall survival vs BSC alone in patients (pts) with aUC that had not progressed with 1L platinum-based induction chemotherapy, and health-related quality of life was maintained. We report long-term and exploratory PRO analyses in the overall avelumab + BSC arm (any treatment duration) and in the subgroup with ≥12 months of avelumab treatment. Methods: In JAVELIN Bladder 100 (NCT02603432), PROs were a secondary endpoint assessed at baseline, on day 1 of each 4-week cycle, at end of treatment/withdrawal, and up to 90 days post treatment. PRO instruments used were National Comprehensive Cancer Network/Functional Assessment of Cancer Therapy Bladder Symptom Index-18 (FBlSI-18) and EuroQol 5 Dimensions 5 Levels (EQ-5D-5L). Descriptive and mixed-effect model analyses were conducted. Data were not evaluated in the BSC alone arm because few pts remained on study treatment at later time points. Results: At data cutoff (Jun 4, 2021), median follow-up in the avelumab arm (n=350) was 38.0 months (≥2 years in all pts), and median duration of treatment was 5.8 months. In pts treated for ≥12 months (n=118 [33.7%]), baseline characteristics were similar to those in the overall avelumab + BSC arm, except for a higher proportion with ECOG performance status of 0 (70.3% vs 60.9%) and lower proportion with visceral metastases (47.5% vs 54.6%). In both populations, completion rates for both PRO instruments among evaluable pts were >80% at all time points during treatment. On average, PRO scores remained stable throughout treatment, and no clinically relevant changes from baseline were reported (Table). Among pts treated for ≥12 months, ≈75% of evaluable pts reported no change or a decrease in being bothered by treatment side effects throughout 24 months of treatment. Conclusions: Prolonged avelumab 1LM treatment (≥12 months) was associated with stable PROs, indicating preservation in quality of life. The results further support the use of avelumab 1LM until progression or unacceptable toxicity as standard of care with level 1 evidence in pts with aUC who are progression-free after platinum-based chemotherapy. Clinical trial information: NCT02603432.