A long-term follow-up study of 42 patients with West syndrome treated with high doses of sodium valproate is presented. Control of the hypsarrhythmic EEG pattern was achieved after two weeks for over three-quarters of the patients with sodium valproate doses of 100 to 300mg/kg/day. Recurrence of hypsarrhythmia was observed most often in patients treated with doses lower than 200mg/kg/day. Other types of seizures appeared in half of the patients followed beyond two years of age. Monotherapy throughout follow-up was possible for 30 patients. Autism occurred in only one infant, and 12 achieved normal mental status. The most common side-effects were asymptomatic thrombocytopenia, vomiting and mild somnolence. Hepatic enzymes were not altered.