Background: Results from the phase 3 JAVELIN Bladder 100 trial (NCT02603432) led international treatment guidelines to recommend avelumab 1L maintenance as a standard-of-care treatment for pts with aUC without progression after 1L platinum-based chemotherapy (PBC). Around 20% of UCs are mixed with other histological subtypes/variants; these tumors do not have specific treatment guidelines and represent an unmet treatment need. In the large real-world AVENANCE study (NCT04822350; N=594) of avelumab 1L maintenance in pts with aUC in France, outcomes in pts with histological subtypes were consistent with those in the overall population. Here, we report a post hoc analysis of long-term outcomes from JAVELIN Bladder 100 in pts with predominantly UC mixed with <50% histological subtype component. Methods: Eligible pts had unresectable locally advanced or metastatic UC without progression after 1L PBC and were randomized 1:1 to receive avelumab + best supportive care (BSC) or BSC alone. The primary endpoint was overall survival (OS) measured from randomization; secondary endpoints included progression-free survival (PFS) and safety. This post hoc analysis was conducted in all pts with histological subtypes. Results: In the avelumab + BSC and BSC alone arms, respectively, 44/350 and 57/350 pts had histological subtypes. At efficacy data cutoff (June 4, 2021), median follow-up in both arms was ≥38.0 mo. In pts with histological subtypes, OS and PFS measured from randomization were prolonged with avelumab + BSC vs BSC alone (Table). Long-term safety (cutoff, April 6, 2023) in treated pts with histological subtypes was generally consistent with the overall safety population. Treatment-related adverse events (TRAEs) of any grade occurred in 36 pts (83.7%) in the avelumab + BSC arm vs 1 pt (1.8%) in the BSC alone arm; grade ≥3 TRAEs occurred in 9 pts (20.9%) and 0 pts, respectively. Conclusions: This exploratory analysis shows the long-term efficacy and safety of avelumab 1L maintenance in pts with histological subtypes in JAVELIN Bladder 100. No new safety concerns were identified. These results were consistent with those in the overall population and support the use of avelumab 1L maintenance in pts with aUC without progression following 1L PBC, including pts with predominantly UC mixed with <50% histological subtype component. Clinical trial information: NCT02603432.