The results are reported of a small clinical trial carried out to assess the potential value of the hypoxic cell radiosensitizer misonidazole in the radiation treatment of Grade 3 and 4 supratentorial astrocytomas. A total of 55 patients were randomly allocated to one of 3 treatment groups. No significant differences were seen between the median survivals of patients in the 2 control radiation groups and that of the third group in which oral misonidazole at a dose of 3 g/m2 preceded each of 4 weekly radiation doses. Possible reasons why no improvement was seen are discussed in detail.