Neurotoxic side effects of misonidazole with peripheral neuropathy was investigated in two series of patients. The first series consisted of eight patients with carcinoma of the pharynx, larynx or lung who, during treatment with misonidazole, developed peripheral neuropathy dominated by severe sensory symptoms and signs localized mainly to the lower extremities. Misonidazole was given for three to seven weeks in a total dose of 9.6 - 12.6 g/m2 (11 g/m2 or more in four of the patients). The symptoms subsided partially within a few months after cessation of the therapy. Electrophysiological and histological findings indicated axonal neuropathy with loss of large fibres and secondary demyelination. The second series consisted of 70 patients with carcinoma of the pharynx or larynx who, in addition to radiotherapy, were given either placebo or misonidazole over four weeks in a total dose of 11 g/m2. Fourteen patients out of 36 receiving misonidazole (38%) developed peripheral polyneuropathy, mostly in the feet, while this occurred in only two of the 34 patient placebo group.