BACKGROUND AND PURPOSE: Radiotherapy (RT) with cetuximab is an alternative for advanced-stage head and neck squamous cell carcinoma (HNSCC) patients who are unfit for cisplatin treatment. As 5-year overall survival is below 50%, it is of interest to test PD-L1 immune checkpoint blockade (avelumab) with cetuximab-RT in the curative setting. MATERIALS AND METHODS: Phase-I feasibility trial (planned n = 10, NCT02938273) of conventional cetuximab-RT with avelumab (concurrent 10 mg/kg Q2W + 4 months maintenance) for advanced-stage HNSCC patients unfit for cisplatin treatment. RESULTS: One of ten included patients experienced grade 2 cetuximab-related infusion reaction and withdrew from the study before avelumab was administered. One patient discontinued treatment after 2 courses of avelumab and 12×2Gy RT for personal reasons. In 2/8 remaining patients, avelumab was stopped after 4 and 8 courses because of toxicity and tumor progression, respectively. There was no grade 4-5 toxicity. Grade 3 immune-related toxicity was manageable and occurred in 4 patients. One patient was treated with topical steroids for grade 3 maculopapular rash and 3 patients received high-dose prednisone for grade 3 elevated liver enzymes (n = 1) and pneumonitis (n = 2). Seven patients experienced grade 3 RT-related toxicity with no severe specific cetuximab-related toxicity. Tumor recurrence occurred in 4/8 patients (50%) after a median of 12 (8-26) months follow-up. CONCLUSION: Cetuximab-RT plus avelumab is feasible in patients with advanced-stage HNSCC who are unfit for cisplatin treatment. Immune-related toxicity was transient and manageable and radiotherapy-related toxicity was in accordance with standard of care. This pilot study provides grounds for larger efficacy trials.