In order to clarify the appropriate usage of adrenocorticotropic hormone (ACTH), the efficacy and side effects of two different regimens of low dose ACTH therapy were compared. Thirty-four patients with West syndrome (WS) were treated with ACTH. The dose of synthetic ACTH-Z was 0.015 mg/kg/dose in 18 patients who were treated between 1996 and 1998 (regimen A), and 0.010 mg/kg/dose in 16 patients who were treated between 1999 and 2001 (regimen B). Patients were classified into cryptogenic and symptomatic groups. Efficacy and adverse effects of ACTH were compared between regimens A and B. Similar analyses were performed after stratification into cryptogenic and symptomatic groups. The efficacy of ACTH was not different between regimens A and B. However, among patients with symptomatic WS, the number of ACTH injections and the dose of ACTH until cessation of spasms were significantly smaller in regimen A than in regimen B. There was no significant difference in these variables between the regimens among those with cryptogenic WS. Adverse effects were not different between regimens A and B. 0.010 mg/kg per day of ACTH will be adequate for cryptogenic WS, but 0.015 mg/kg per day of ACTH is recommended for symptomatic WS.