The risk of late epilepsy was analyzed in a consecutive series of 177 patients operated on for supratentorial aneurysms. Late seizures occurred in 25 patients (14%); the seizures were recurrent in 21 patients (12%). Most seizures were partial, secondary generalized, or generalized tonic-clonic (72%). The mean latency between the operation and seizures was 8.4 months (range, 1 to 24 months), and in only 2 patients was the interval more than 12 months. The most important risk factors were preoperative and postoperative complications. Only 2.5% of the 81 Grade I patients developed epilepsy, compared to 33% of the 42 Grade III-IV patients. Other risk factors were location of the aneurysm in the middle cerebral artery, the presence of a large intracerebral hematoma, postoperative spasm with late ischemic infarction, and shunt-dependent hydrocephalus. The timing of operation or intraventricular intracranial pressure monitoring did not change the risk of late epilepsy. The fact that only 2 patients had early epilepsy may have been due to routine treatment with prophylactic anticonvulsants. The value of prophylaxis in late epilepsy is unproven, but prophylactic treatment could be useful in patients with a high risk of epileptic seizures.