Misonidazole, a 2-nitroimidazole, is receiving extensive trial as a radiosensitizing drug in the treatment of patients with malignant disease. Drug sensitivity rashes have been observed and the collected cases from three centers are reported. Of a total of 380 patients there were five (1.3%) instances of definite rash and ten (2.6%) instances of observed rash. These figures are compared with the incidence of sensitivity rashes with other drugs in the nitroimidazole group.