The authors review the seizure incidence in 4232 adult patients with mild closed head injury who did not receive prophylactic anticonvulsant agents. One hundred patients (2.36%) experienced seizures within 1 week after head injury; 43 of these (1.02% of the series) had seizures within 24 hours after trauma. Most of the seizures (84%) that developed during the 1st week after injury were of the generalized tonic-clonic type. The incidence of generalized tonic-clonic seizures was higher than that of partial seizures with motor symptoms both within 24 hours (91% vs. 9%) and during the Day 2 to 7 period (79% vs. 21%). No definite intracranial pathological findings were detected by computerized tomography (CT) in 53% of patients with early posttraumatic seizures; six patients had intracranial hemorrhage without intracranial parenchymal damage (three with epidural hematoma and three with subarachnoid hemorrhage). The most common positive CT findings in the early posttraumatic-seizure group were intracerebral hemorrhage (24%), followed by acute subdural hematoma with intracerebral hemorrhage (17%). Intracerebral parenchymal damage could be identified on CT scans in 41 (48.8%) of 84 patients with generalized tonic-clonic seizures and five (31%) of 16 patients with partial seizures with motor symptoms. The intracerebral parenchymal damage was most commonly detected in the frontal lobe (21%) and the temporal lobe (19%). Seven patients with early posttraumatic seizures received emergency craniotomy to remove an intracranial hematoma (epidural in three, subdural and intracerebral in four) because the mass effect resulted in significant midline shift as seen on CT scans. This review suggests that early posttraumatic seizures after mild closed head injury have a high incidence (53%) in patients with normal CT scan findings. Although the possibility of surgically correctable intracranial hemorrhage is low (7%), the condition may be devastating if not treated properly.