Background: Merkel cell carcinoma (MCC) is a rare, aggressive skin cancer. Avelumab, a human anti–PD-L1 monoclonal antibody, is FDA and EMA approved for the treatment of metastatic MCC (mMCC). Here, we report efficacy and safety data for avelumab in patients (pts) with mMCC at ≥18-mo of follow-up. Methods: Pts with mMCC and progression on prior chemotherapy received avelumab 10 mg/kg IV Q2W. Objective response rate (ORR), duration of response (DOR), and progression-free survival (PFS) were evaluated by independent review committee (RECIST v1.1) with tumor assessment every 6 wks; overall survival (OS) and adverse events (AEs; NCI CTCAE v4.0) were also evaluated. Results: As of Mar 24, 2017, 88 pts were treated and followed for a median of 23.0 mo (range 18.7–32.0). Median treatment duration was 17 wks (range 2–132; mean 35 wks ±37). Treatment was ongoing in 15 pts (17%); 7 pts (8%) voluntarily discontinued with continuing responses (5 complete responses [CR] and 2 partial responses). Other reasons for discontinuation were disease progression (n = 42; 48%), death (n = 10; 11%), AE (n = 8; 9%), or consent withdrawal/other (n = 6; 7%). Confirmed ORR of 33% (95% CI 23.3–43.8; CR in 11.4%) remained unchanged from previous analysis and median DOR has not been reached (range 2.8–24.9 mo; 95% CI 18.0–not estimable). Responses were ongoing in 20/29 pts (69%), including 5 pts with > 2 y of follow-up. PFS curve showed a plateau with identical 12- and 18-mo rates of 29% (95% CI 19–39). Median OS was 12.6 mo (95% CI 7.5-19.0) and the 12- and 18-mo OS rates were 51% (95% CI 40–61) and 40% (95% CI 29–50), respectively. 66 pts (75%) had a treatment-related (TR)AE, most commonly ( > 10%) fatigue (25%), infusion-related reaction (15%; all grade 1/2), nausea (11%), and diarrhea (11%); 8 (9%) had a grade ≥3 TRAE. 17 pts (19%) had an immune-related AE, which was grade ≥3 in 4 pts (4.5%). No treatment-related deaths occurred. Conclusions: Updated results demonstrate continued durable antitumor activity of avelumab in pts with mMCC, beyond that expected with cytotoxic chemotherapy. The median OS exceeding 1 y and manageable safety profile of avelumab signify a clinically meaningful benefit in pts with mMCC. Clinical trial information: NCT02155647