Abstract Introduction: Chemotherapy can potentially enhance the activity of immune checkpoint inhibitors by promoting immune priming. The phase 1b/2 JAVELIN Chemotherapy Medley trial evaluated first-line avelumab + concurrent chemotherapy in patients with advanced urothelial carcinoma or nonsmall cell lung cancer (NSCLC). Patients and methods: Avelumab 800 mg or 1200 mg was administered continuously every 3 weeks (Q3W) with standard doses of cisplatin + gemcitabine in patients with urothelial carcinoma, or carboplatin + pemetrexed in patients with nonsquamous NSCLC. Dual primary endpoints were dose-limiting toxicity (DLT; phase 1b) and confirmed objective response (phase 1b/2). Results: In phase 1b, urothelial carcinoma and NSCLC cohorts received avelumab 800 mg (n=13 and n=6, respectively) or 1200 mg (n=6 each) + chemotherapy. In evaluable patients with urothelial carcinoma treated with avelumab 800 mg or 1200 mg + chemotherapy, DLT occurred in 1/12 (8.3%) and 1/6 (16.7%), respectively; no DLT occurred in the NSCLC cohort. In phase 2, 35 additional patients with urothelial carcinoma received avelumab 1200 mg + chemotherapy. Across all treated patients, safety profiles were similar irrespective of avelumab dose. Objective response rates (95% confidence internal) with avelumab 800 mg or 1200 mg + chemotherapy, respectively, across phase 1b/2, were 53.8% (25.1-80.8) and 39.0% (24.2-55.5) in urothelial carcinoma, and 50.0% (11.8-88.2) and 33.3% (4.3-77.7) in NSCLC. Conclusions: Preliminary efficacy and safety findings with avelumab + chemotherapy in urothelial carcinoma and NSCLC were consistent with previous studies of similar combination regimens. Conclusions about clinical activity are limited by small patient numbers. ClinicalTrials.gov identifier, NCT03317496