We compared DNA damage and the efficacy of its repair after genotoxic treatment with γ-radiation of lymphocytes and tissue cells isolated from patients with squamous cell carcinoma of head and neck (HNSCC) and healthy donors. Thirty-seven subjects with HNSCC and 35 healthy donors were enrolled in the study. The extent of DNA damage including oxidative lesions and efficiency of the repair were examined by alkaline comet assay. HNSCC cancer cells were more sensitive to genotoxic treatment and displayed impaired DNA repair. In particular, lesions caused by γ-radiation were repaired less effectively in metastasis of HNSCC than in healthy controls. The differences in radiation sensitivity of cancer and control cells suggested that DNA repair might be critical for HNSCC treatment. We conclude that γ-radiation might be considered as an effective therapeutic strategy for head and neck cancers, including patients in advanced stage of the disease with clear evidence of metastasis.