A long-term prospective study was carried out of 133 children diagnosed as having epilepsy in the first year of life, of whom two-thirds had West syndrome and one-third had other forms of epilepsy. They were followed for a minimum of three years (half for over seven years), during which time 15 children died. Of the 118 surviving, 54 had an IQ of > 70, but 53 were severely mentally impaired, of whom two-thirds had West syndrome. Only 56 per cent currently have no seizures, and no significant differences were found in this respect between children with West syndrome and those with other forms of epilepsy. Regression in mental development occurred significantly more frequently among children with active epilepsy. These results lead to the conclusion that the degree and type of central nervous system damage existing at the onset of epilepsy is decisive for the outcome of the child, but the cessation of epileptic seizures also improves the child's developmental possibilities.