OBJECTIVE To determine the incidence and risk factors for seizure recurrence after the onset of late posttraumatic seizures (ie, seizures occurring more than 7 days after injury). METHODS Longitudinal cohort design. METHODS Level 1 trauma center. METHODS Sixty-three moderately to severely head-injured adults who developed late posttraumatic seizures during the course of their participation in a randomized, placebo-controlled study of the effectiveness of prophylactic phenytoin (Dilantin) for prevention of posttraumatic seizures. METHODS Time from the first unprovoked late seizure to time of seizure recurrence. RESULTS The cumulative incidence of recurrent late seizures was 86% by approximately 2 years. However, the frequency of recurrent seizures varied considerably across subjects: 52% experienced at least five late seizures, and 37% had 10 or more late seizures within 2 years of the first late seizure. The relative risk of recurrence was highest in patients with a history of acute subdural hematoma and prolonged coma (ie, longer than 7 days). CONCLUSIONS When late seizures develop after severe head injury, the probability of recurrence is high, which suggests that patients be treated aggressively with anticonvulsant medication after a first unprovoked late seizure.