We studied eight patients with carcinoma of the pharynx and larynx (five cases) or lungs (three cases) who, during treatment with the radiosensitizing drug misonidazole, developed peripheral neuropathy dominated by severe sensory symptoms and signs mainly localized to the lower extremities. The symptoms partially subsided within months after cessation of therapy. Electrophysiological and histological findings indicated an axonal neuropathy with loss of large fibers and secondary demyelination. The neurotoxic property of misonidazole limits its therapeutic use.