Background: The programmed death-1 receptor (PD-1) and its ligand (PD-L1) are key therapeutic targets in the reactivation of the immune response against multiple cancers. Avelumab* (MSB0010718C) is a fully human anti-PD-L1 IgG1 antibody currently being investigated in phase I/II trials. Here we report on the safety and tolerability of avelumab in a phase I dose escalation study (NCT01943461) in Japanese patients (pts) with advanced solid tumors. Methods: Dose escalation (3+3 design) was performed for 3 dose levels (DL 3, 10, and 20 mg/kg). The dose limiting toxicity (DLT) evaluation period was 3 weeks. Results: A total of 17 pts (4 gastric, 4 melanoma, 3 NSCLC, 2 CRC, 2 GIST, 1 esophageal, 1 MBC) were enrolled, with 5, 6, and 6 pts treated with avelumab at 3, 10, and 20 mg/kg, Q2W, respectively. Median age was 62 years (range 30–74) and all had ECOG PS 0 or 1, with a median of 4 prior lines of therapy (range 1–9). Treatment-emergent adverse events (TEAEs) occurred in 16/17 pts (94.1%), of which 11 (64.7%) were treatment-related. The most frequent related TEAEs (≥ 10%) were infusion-related reactions (5, 29.4%), stomatitis (4, 23.5%), maculopapular rash (4, 23.5%), decreased white blood cell count (3, 17.6%), pyrexia (2, 11.8%), headache (2, 11.8%), and anemia (2, 11.8%). Two pts had grade 3 TEAEs (not related). No pt experienced a DLT. Two pts had confirmed partial responses (1 melanoma, 1 esophageal), and 5 had stable disease for > 3 months. To date, 12 pts (70.6%) have discontinued treatment: 10 (58.8%) for progression, 1 (5.9%) for death, and 1 (5.9%) for other reason. Five pts (29.4%) remain on treatment. All 17 pts were evaluable for PK analysis. Half-life was 88.6, 121.8, and 114.0 h for dose levels 3, 10, and 20 mg/kg, respectively. The parameters C_max and AUC were approximately proportional with dose, showed low variability, and were similar to those previously obtained in Caucasian pts. Conclusions: Avelumab can be safely administered to Japanese pts in doses up to 20 mg/kg Q2W. Additional phase II/III studies to evaluate the clinical activity of avelumab at 10 mg/kg Q2W in select tumor types are currently being planned. *Proposed INN. Clinical trial information: NCT01943461