OBJECTIVES: Patient-reported outcomes are an important component of drug benefit/risk and reimbursement evaluation, but limited data are available from patients with rare tumors, such as Merkel cell carcinoma (MCC). This study aimed to assess differences in health utility scores between non-progressing and progressing patients with metastatic MCC treated with anti–PD-L1 avelumab. METHODS: EQ-5D data collected from a phase 2 single-arm trial (NCT02155647) of 88 patients with chemotherapy-treated metastatic MCC were analyzed. The EQ-5D was assessed at baseline, week 7, every 6 weeks thereafter, and at the end-of-treatment visit. At each assessment, tumor response was determined by radiologically by an independent review committee per RECIST v1.1 performed within approximately 7 days of the EQ-5D assessment. EQ-5D utilities were calculated based on US and UK value sets. Linear mixed models were fitted to EQ-5D data, including progressive disease (vs complete response/partial response/stable disease) as a single time-varying covariate. In sensitivity analyses, estimates were adjusted for grade 3-4 adverse events (AEs) ongoing at EQ-5D assessment and treatment-related AEs of any grade. RESULTS: Among 70 evaluable patients, 247 observations were analyzed. Utility based on the US (UK) value sets was 0.8058 (0.8327) in the non-progression health state and 0.7120 (0.7130) in the progression health state. Differences between health states were statistically significant (p<0.0001) and clinically relevant. Adjusting for the presence of AEs had minimal impact. Parameter estimates of dis-utilities associated with experiencing ≥1 grade 3-4 AE based on US (UK) value sets were –0.02191 (–0.02439). Dis-utilities for treatment-related AEs of any grade were smaller (–0.00532 and –0.01028 for US and UK value sets). CONCLUSIONS: In patients with metastatic MCC, non-progression during treatment with avelumab contributed to gains in health utility scores. The dis-utility impact of AEs during treatment with avelumab was minimal, suggesting a manageable safety profile from a patient perspective.