To provide up-to-date information on adrenocorticotropic hormone (ACTH) therapy in the treatment of West syndrome, a review of the Finnish studies was made in answer to the questions: what are (1) its efficacy: doses and comparison with vigabatrin (VGB), (2) its tolerability, (3) its mechanism of action? Why do some patients respond, but others do not? No other drugs have been shown to be more effective than ACTH. High doses were not more effective than low doses. Synthetic derivatives were associated with more frequent side effects. Individualized therapy was developed on the basis of etiology and response. With therapy consisting of ACTH 3-6IU/kg/day, all the cryptogenic and half of the symptomatic spasms could be controlled within over 2-3 weeks therapy and with minimal risk of side effects. In a Finnish study, 26% of the patients responded to VGB as the first-line drug. Some of the non-responders responded to ACTH. In tuberous sclerosis, the initial response rate to ACTH was high (73%) and did not differ from the response rate to VGB in other series. Both drugs have severe side effects. The visual field defects caused by VGB occur even in children (in 18/91 Finnish children). The patients with cryptogenic spasms, who responded well to ACTH, differed in their biochemical parameters from the patients with symptomatic spasms. The therapeutic action of ACTH may be mediated by potentiation of nerve growth promoting activity. Neurodegeneration may be due to imbalance between nerve growth factors and nitrate/nitrite in the brain. ACTH should be used as the first choice for treatment of West syndrome (at the minimal effective dose and for shortest effective time). The side effects of steroids, unlike VGB, are well known, treatable, and reversible.