The prognosis of infantile spasms is grim when a detectable brain lesion is present. In contrast, cryptogenic infantile spasms, in which there is no identifiable brain lesion, usually run a favorable course under treatment. Few studies have focused on the outcome in children with cryptogenic infantile spasms. Among 111 pediatric patients with a history of infantile spasms hospitalized over 18 consecutive years, 23 (21%) were given a diagnosis of cryptogenic infantile spasms. Follow-up ranged from 4 to 21 years. Outcome was as follows: the IQ was above 80 in 39% of cases and above 100 in 13% of cases; 42% of patients of school age were attending school but half of these had learning disabilities; 30% of patients had severe psychiatric disorders, and 22% had developmental delay and severe epilepsia. Early factors apparently associated with a good prognosis included the mild nature of psychomotor regression, persistence of spindles on EEGs recorded during NREM sleep, and prompt improvement of clinical status and EEG recordings under treatment. Conversely, severe regression, focalized EEG anomalies, failure of development to resume promptly after initiation of therapy, recurrence of spasms and hypsarrhythmia at discontinuation of treatment, and onset before 5 months or after 11 years of age were associated with a poor prognosis.