We determined the average annual incidence of epilepsy characterized by infantile spasms (IS) in Iceland based on the 13 cases identified in a 10-year interval (1981-1990). The cumulative incidence was 3 in 10,000 live births. Males were more frequently affected than females. All patients responded completely (10) or partially (3) to ACTH or steroid therapy. At follow-up, all children with cryptogenic IS are seizure-free and have developed normally intellectually. One of the children with cryptogenic IS has mild spastic diplegia. All children in the symptomatic group are mentally retarded, and 5 of 7 continue to have unprovoked seizures. We detected no evidence for familial aggregation, and the incidence of seizures or epilepsy did not appear to be increased in first-degree relatives of cases. The proportion of cases with a favorable outcome was greater than that reported in most clinical series.