Thirty-two patients with a presenting complaint of petit mal seizures and with eeg verification of the seizures were followed after an average interval of 18 years from the onset of the petit mal seizures. Information was obtained on the present status of all patients. Twenty-one were seen personally and given a neurologic examination. In 11 patients, it was possible to obtain a repeat eeg. Thirteen of the patients continued to have petit mal seizures at the time of the follow-up examination, although the frequency and duration of the seizures was greatly reduced. Five patients had petit mal and grand mal at the time of the follow-up examination, 2 patients had grand mal only at the time of the follow-up, and 12 patients were having no seizures at the time of the follow-up examination. A total of 12 of the 32 had experienced grand mal after the onset of the petit mal and, in 5 of these, it had ceased spontaneously or was under complete control with medications. None developed psychomotor seizures. Therefore, up to an average age of 25, 38% of these series had experienced grand mal which, in half of the cases, ceased spontaneously or was easily controlled; 31% had remission of their petit mal and never developed grand mal; and 56% continued to have petit mal, usually without grand mal, the petit mal in most cases being minimal and very little trouble to the patient. If the patient reached the age of 21 with petit mal, it appeared to continue indefinitely (within the limits of this follow-up) and, if the patient had not experienced grand mal by the age of 18, it did not develop later. It was difficult to predict by any means whether a patient would continue to have petit mal or would experience grand mal seizures, but favorable prognostic signs may include early onset (between 3 and 10 years), more than 10 petit mal spells a day when at the most frequent, positive family history, normal mentality, and a normal neurologic examination. Favorable eeg findings relative to nonoccurrence of grand mal appeared to be posterior emphasis of the spike-wave discharges and single rather than multiple spike discharges. Unfavorable eeg findings (relative to future grand mal) appear to be a slow basic pattern and slow (2 to 3 per second) spike-wave bursts. There was no evidence of progressive physical or mental deterioration in any of the patients except 1 in whom multiple sclerosis developed. "Pure" petit mal appears to be a relatively benign disorder which occurs twice as often in girls as in boys, is associated with a family history of grand mal seizures in half the cases, occurs usually between the ages of 3 and 13, more often than not continues into adult life, usually does not bring on grand mal seizures, is not transformed into psychomotor seizures, and is not associated with mental or neurologic deficits or degeneration.