The incidences of postoperative seizures and side effects were evaluated in 193 patients with cerebral aneurysm who received anticonvulsant prophylaxis and underwent 224 craniotomies for cerebral aneurysms between 1993 and 1995. The patients were 73 males and 120 females aged between 31 and 80 years. One hundred and sixteen patients had ruptured cerebral aneurysms and 108 had unruptured aneurysms. Phenytoin followed by valproic acid were administered. Early postoperative seizures occurred in five patients (4 with ruptured and 1 with unruptured aneurysms) within 14 days after surgery. Late postoperative seizures occurred in four different patients with ruptured aneurysms more than 14 days after surgery. The presence of cortical lesions detected by cerebral computed tomography and occurrence of symptomatic vasospasm were correlated with the occurrence of early postoperative seizure (p < 0.05). Three of the four patients with late postoperative seizure had cortical lesions and two were receiving continued medication. Side effects that warranted discontinuation of therapy were seen in the follow-up period in 12.9% of patients. Anticonvulsant prophylaxis is not recommended due to the higher incidence of side effects than seizure, except in patients in poor clinical condition for the purpose of brain protection. Otherwise, anticonvulsant medication should be initiated at the time of the initial seizure attack.