Adrenocorticotrophic hormone (ACTH) therapy is an established treatment for West syndrome. However, some patients may relapse after this therapy, for whom there is no established treatment. We describe 3 patients with symptomatic West syndrome and multiple, poor prognostic factors who relapsed after initial ACTH therapy. They were treated with a second round of ACTH therapy, i.e., daily intramuscular injection for 2-3 weeks and subsequent withdrawal, alternative days for 1 or 2 weeks, every 3 days for 1 or 2 weeks, followed by weekly or biweekly for >/=1 year. Clinical seizures and hypsarrhythmia resolved in all 3 patients within 4 weeks, and these clinical improvements continued through a second round of ACTH therapy. Two patients developed other types of seizures and aggravation of paroxysms on electroencephalography, but no hypsarrhythmia, soon after termination of ACTH therapy. In the other patient, although electroencephalographic findings remained normal during weekly ACTH therapy, focal sharp waves with irregular slow waves appeared after the injection interval became biweekly. After a second round of ACTH therapy, all patients exhibited developmental progress, particularly in gross motor development and visual functions. No serious adverse events occurred during treatment. Long-term weekly ACTH therapy is a potentially effective treatment option for relapsed West syndrome.