The frequency with which polyneuropathy developed was investigated in patients with cancer of the larynx and pharynx who participated in a double-blind trial of the radiosensitizing drug misonidazole. Fourteen of 36 patients receiving misonidazole (total dose of about 11 g/m2) developed neuropathy, while this occurred in only 2 of 34 patients in the placebo group. Vibration perception threshold increased in all patients who developed neuropathy, but also in 12 (5 misonidazole and 7 placebo treated) without other symptoms or signs of neuropathy. Pharmacokinetic studies of misonidazole revealed a correlation between development of neuropathy and a high 'peak plasma concentration/g misonidazole in each fraction' and especially a high 'area under plasma concentration curve/g misonidazole in each fraction'.