Adrenocorticotropic hormone (ACTH) therapy is a widely used and effective treatment for West syndrome. However, serious adverse effects can occur during or shortly after ACTH therapy. Synthetic ACTH has been reported to cause more adverse effects than natural ACTH. Currently, most Japanese pediatric neurologists try other non-hormonal treatments before using ACTH therapy, but even then, they use extremely low dosages of synthetic ACTH. The extremely low-dose synthetic ACTH therapy currently used in Japan is as effective for treatment of West syndrome as the higher doses reported in previous studies from Japan and Western countries. The dosage and duration of synthetic ACTH therapy should be as low as possible to avoid serious adverse effects. Close monitoring of the adverse effects, especially subdural hematoma, is necessary even when extremely low dosages of ACTH are administered.