To study the efficacy of adrenocorticotrophic hormone (ACTH) in treating Taiwanese children with West syndrome (WS) and the impact on long-term prognosis, 66 patients with WS (54 symptomatic and 12 cryptogenic) were collected from 1987 to 1998 in a medical center in Taiwan. A total of 53 patients were enrolled in this study and treated with ACTH at the dosage of 2.5IU/kg daily for 2 weeks with gradual tapering in subsequent 6 weeks. Immediate responses, side effects of ACTH and long-term outcomes of the patients including seizure and developmental status were evaluated during the average follow-up period of 35.6 months. The spasm-free percentage after one or two courses of ACTH treatment was 77.4%. Nine (17%) patients encountered severe side effects such as major infections, which prompted us to stop ACTH. At the end of follow-up, 22 (41.5%) patients had intractable seizures but 25 (47.2%) patients remained seizure free with or without anticonvulsants. The ACTH-responders had a better chance of remaining seizure free (P<0.05). Regarding the long-term developmental outcome, 12 (22.6%) patients had normal or borderline development; two thirds of them belonged to the crytpogenic group. Six (11.3%) patients expired and 24 (45.3%) were severely retarded; all but one of them belonged to the symptomatic group. The prognosis of WS heavily relies on whether a patient is cryptogenic or symptomatic (P<0.001). Good response to therapy or short treatment lag did not favorably affect the developmental outcomes of the symptomatic cases. We conclude that the long-term outcomes of WS in Taiwan were generally poor despite of treatment. Only cryptogenic patients had favorable prognosis. For symptomatic patients, ACTH therapy may be used to control the spasms and decrease the incidence of subsequent epilepsy, but it will not improve developmental outcome. Considering a high percentage of severe side effects in our study, a lower dosage of ACTH with adequate therapeutic efficacy but less side effects should be considered for treating Taiwanese children with WS.