Background: The phase 3 JAVELIN Bladder 100 trial, which enrolled patients (pts) with advanced UC that had not progressed with 1L platinum-containing chemotherapy, showed that maintenance therapy with avelumab + best supportive care (BSC) significantly prolonged overall survival (OS) compared with BSC alone (hazard ratio [HR], 0.69 [95% CI: 0.56, 0.86; 1-sided P= 0.0005]). However, the optimal timing for starting avelumab after completing 1L chemotherapy is unknown. In this post hoc analysis, we report efficacy by duration of the TFI from completion of 1L chemotherapy. Methods: In the JAVELIN Bladder 100 trial (NCT02603432), eligible pts had unresectable locally advanced or metastatic UC without disease progression following 4 to 6 cycles of 1L platinum-containing chemotherapy. Pts were randomized to receive avelumab + BSC (n = 350) or BSC alone (n = 350) after a TFI of 4 to 10 weeks from the last dose of chemotherapy. In this exploratory analysis, subgroups with a TFI of 4 to < 6 weeks ( < 42 days), 6 to < 8 weeks (42 to < 56 days), or 8 to 10 weeks (≥56 days) were evaluated. Results: In the avelumab + BSC and BSC alone arms, the TFI was 4 to < 6 weeks in 143 and 158 pts, 6 to < 8 weeks in 109 and 80 pts, and 8 to 10 weeks in 98 and 110 pts, respectively. Baseline characteristics in these subgroups were generally well balanced between arms. For both arms combined, however, the TFI 4 to < 6 weeks subgroup vs the other 2 subgroups included more pts with visceral metastases (57.8% vs 54.0% and 50.0%), an objective response with 1L chemotherapy (76.4% vs 69.3% and 68.3%), and an ECOG performance status of 1 (44.5% vs 33.3% and 35.6%). OS was prolonged with avelumab + BSC vs BSC alone in all subgroups; the HR was 0.76 (95% CI: 0.546, 1.059) in the TFI 4 to < 6 weeks subgroup (median OS, 19.9 months [95% CI: 16.3, 25.3] vs 13.5 months [95% CI: 11.7, 17.4]), 0.64 (95% CI: 0.404, 1.021) in the TFI 6 to < 8 weeks subgroup (median OS, 26.1 months [95% CI: 19.9, not estimable] vs 21.0 months [95% CI: 10.7, not estimable]), and 0.70 (95% CI: 0.468, 1.035) in the TFI 8 to 10 weeks subgroup (median OS, 20.1 months [95% CI: 13.8, not estimable] vs 14.1 months [95% CI: 11.7, 19.6]). Conclusions: In patients with advanced UC that had not progressed with 1L platinum-containing chemotherapy, avelumab 1L maintenance prolonged OS irrespective of the TFI assessed in this study (4-10 weeks), supporting this new treatment strategy as a standard of care. Differences in duration of TFI were likely related to individual patient- and disease-specific characteristics or logistics and did not impact the OS benefit observed with avelumab 1L maintenance. Clinical trial information: NCT02603432