Background: Avelumab is a human anti‒PD-L1 IgG1 antibody approved in the US, Canada, and Israel for the treatment of locally advanced or metastatic urothelial carcinoma (mUC) progressed after platinum chemotherapy. In the JAVELIN Solid Tumor study, avelumab treatment in post-platinum patients (pts) has shown promising and durable antitumor activity and favorable overall survival (OS) outcomes. Here, we report post hoc analyses to assess if early response to avelumab is associated with long-term treatment outcomes. Methods: Pts with mUC that had progressed after platinum-based therapy in the JAVELIN Solid Tumor study were analyzed. Tumors were assessed every 6 weeks by independent review (RECIST v1.1). A Kaplan-Meier subgroup analysis has been performed for duration of response (DOR); response by week 7 (first tumor scan ± 5 days) define balanced subgroups. A landmark analysis was used to assess the association between time-to-response and OS. The landmark was 3 mo (second tumor assessment; 12 weeks ± 5 days). Results: 242 pts with platinum-treated mUC received avelumab and were followed up for ≥2 years (median, 2.7 years; data cutoff, Apr 2018). No difference in DOR was found between pts who had responded by the first tumor assessment (n=15) and those who responded later (n=25; HR, 0.8 [95% CI: 0.3-2.0]). In a landmark analysis for OS, the median OS was not reached (NR [95% CI: 18.9 mo-NR]) in pts who had responded within 3 mo (n=29) compared with 9.5 mo (95% CI: 6.5-14.4) in pts with late response or stable disease (n=74), and 4.3 mo (95% CI: 2.8-7.1) in other pts (progressive disease [PD] or not evaluable; n=70). Further sensitivity analyses found that survival did not vary between pts who continued treatment beyond investigator-confirmed PD (n=46) vs those that did not (n=24; median survival, 5.0 mo [95% CI: 2.8-8.8] vs 4.3 mo [95% CI: 0.9-8.3], respectively). Conclusions: Responses to avelumab have similar durability irrespective of time to response. Pts with an early response to avelumab tend to survive longer than pts with stable disease, and both groups have longer OS than nonresponders. Clinical trial information: NCT01772004