This prospective study was undertaken to evaluate the risk of epilepsy after aneurysm operations. The patients were discharged after operation without any anticonvulsant prophylactic treatment and followed-up for 12 months. Out of 128 such patients 121 were submitted for final evaluation. Epilepsy was diagnosed if two or more seizure attacks occurred during that time. Such attacks occurred in 8 patients, so the risk of epilepsy was estimated at 7% for the 12 months after operation in patients without prophylactic treatment. In another 3 patients single seizures occurred during the follow-up, they were not treated with anticonvulsant drugs; seizures did not recur for up to two years. Detailed analysis of the patients with late epilepsy revealed that most of them were pre-operatively in the 3rd clinical group according to WFNS scale. The rationale for the use of prophylactic anticonvulsants after aneurysm surgery seems to be doubtful in view of this study and data from the literature.