Ten infants with infantile spasms (IS) and hypoarrhythmic EEG, received 0.5 mg i.m. of synthetic ACTH, three times a week for 2 weeks. Six of them showed complete remission of seizures and dramatic EEG changes, and 3 cases resulted in a partial response. Spasms were controlled during the first week in 5/6 cases where treatment was successful. A significant relationship was observed between treatment precocity and favourable outcome (Fisher p < 0.02). Synthetic ACTH treatment did not produce severe side effects in any patient. During follow up (X 6.2 m) relapse occurred in one case, with 5 patients maintaining the initial positive response and a normal or mildly retarded psychomotor development. The other five patients show severe psychomotor retardation and intractable spasms and/or other seizures. Future research should focus on the efficacy of other treatment schedules, equilibrating success and side effects.