In order to more precisely define a syndrome of medial temporal lobe epilepsy, histories and physical findings were evaluated in 67 patients studied with intracranial electrodes who had medial temporal seizure onset and became seizure free following temporal lobectomy. Patients with circumscribed, potentially epileptogenic mass lesions were excluded. Fifty-four patients (81%) had histories of convulsions during early childhood or infancy, 52 of which were associated with fever. Complicated febrile seizures occurred in 33 (94%) of the 35 patients in whom detailed descriptions of the febrile seizures were available. Bacterial (5) or viral (2) central nervous system infections were present in 7 patients with seizures and fevers. Other less common, but probably significant, risk factors included head trauma (10%) and birth trauma (3%). Only 5 patients had no apparent risk factors. The mean age at habitual seizure onset was 9 years. All patients had complex partial seizures, with half having only complex partial seizures. The other half also had secondarily generalized tonic-clonic seizures, but these were never the predominant seizure type. Only 3 patients had histories of convulsive status epilepticus and no patient had a history of nonconvulsive status epilepticus. All but 3 patients reported auras before some or all of their seizures, with an abdominal visceral sensation being by far the most common type of aura (61%). Of the 60 patients with identified risk factors, all but 2 had an interval between the presumed cerebral insult and the development of habitual seizures, with a mean seizure-free interval of 7.5 years. Fifteen patients developed a seizure-free interval following onset of habitual seizures, with a mean duration of 5.9 years. In 22 patients, the seizures had an evolutionary pattern, with seizures becoming progressively more elaborate over time. We conclude (1) there is a very strong relationship between complicated febrile seizures during early childhood or infancy and the later development of medial temporal lobe epilepsy; (2) habitual seizures in most patients from this population begin during, or shortly after, the first decade of life; (3) complex partial seizures are seen in all such patients; (4) generalized tonic-clonic seizures are not a predominant seizure type in these patients, and convulsive status epilepticus in uncommon; (5) nonconvulsive status epilepticus rarely, if ever, occurs in these patients; (6) auras, particularly abdominal visceral sensations, are very common; and (7) medial temporal lobe epilepsy can be a progressive disease as evidenced by silent intervals and progressive elaboration of seizures.