One hundred thirty-six patients with epilepsy, mostly children, were treated with Celontin® for periods ranging from 6 to 15 months. The maximum dosage of one or more other anticonvulsant agents administered for periods of 1 year or more had previously proved inffective in controlling seizures in 85 of these patients. Celontin® was effective in controlling minor motor and psychomotor seizures in this group of patients. The frequency of spells was reduced in some of the patients with petit mal epilepsy. Patients with major motor seizures were not significantly benefited by this drug. Side effects were experienced by 18 of the 136 patients treated. Drowsiness was the most frequently occurring side effect. There were no irreversible toxic disturbances.