A retrospective study was performed to evaluate the efficacy of prophylactic anticonvulsants in preventing seizures in 68 patients with supratentorial astrocytomas who had been treated with operation and irradiation and who had no previous history of convulsions. Thirty-three patients received prophylactic anticonvulsants and 38 patients did not. The incidence of all types of seizures (generalized convulsions or partial) was lower in patients receiving anticonvulsants. No seizures with an impairment of consciousness occurred in the patients with documented therapeutic anticonvulsant blood levels. The overall incidence of seizures was 39% in untreated patients and 21% in treated patients. The incidence of major seizures including tonic/clonic or partial complex seizures with impairment of consciousness was zero in patients with therapeutic anticonvulsant levels and 18% in untreated patients. Regarding the overall incidence of seizures in both groups, there tend to be fewer seizures in older patients, females, patients with a higher grade of malignancy, and patients who had a more radical resection of the tumor. This study suggests that seizures are a frequent occurrence after operation and irradiation for supratentorial glioma and that anticonvulsants may be effective in reducing the incidence of those seizures.