Sixty adult patients with incompletely excised low-grade gliomas were randomly assigned to receive radiotherapy (55 Gy over a total of 6 1/2 to 7 weeks) either alone or with 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU; 100 mg/sq m every 6 weeks). Pathological review showed that six patients were ineligible for the study. Evaluation of patient age, extent of surgery, tumor grade, and performance status showed no significant differences between the treatment arms. The response rate, as judged by the disappearance or reduction in size of the tumor on computerized tomography scans, was 79% for radiation therapy alone versus 54% for irradiation plus CCNU. The median survival time was 4.45 years for all patients, with no significant difference between treatment arms (p = 0.7). For the group as a whole, patient age and performance status were the most important prognostic parameters. The majority of patients receiving chemotherapy experienced moderate hematological toxicity. This study demonstrates that CCNU chemotherapy does not improve the results of radiation therapy in the treatment of incompletely excised low-grade gliomas.