Among 45 patients with cryptogenic West Syndrome (WS) we report 30 with a favorable outcome consisting of normal psychomotor development and cessation of epilepsy with at least 2 years follow-up (mean 4 years 7 months). These favorable patients could be recognized from onset by (a) absence of significant mental regression with a preserved visual function; (b) absence of focal interictal EEG abnormalities after intravenous diazepam; and (c) reappearance of hypsarrhythmia between consecutive spasms of a cluster. The latter criterion requires EEG recording of a series of spasms. The favorable outcome in these patients suggests that they had no cortical brain lesions. This new type of idiopathic epilepsy referred to as idiopathic West syndrome has important prognostic, therapeutic, and etiologic implications.