Of 421 veterans who had penetrating brain wounds in Vietnam 15 years ago, 53% had posttraumatic epilepsy, and one-half of those still had seizures 15 years after injury. The relative risk of developing epilepsy dropped from about 580 times higher than the general age-matched population in the first year to 25 times higher after 10 years. Patients with focal neurologic signs or large lesions had increased risk of epilepsy, and site of the lesion may have been more important than size in determining occurrence. Family history of epilepsy or preinjury intelligence had no effect on seizure occurrence. Seizure frequency in the first year predicted future severity of seizures. Phenytoin therapy in the first year after injury did not prevent later seizures.