To minimize adverse effects and to get good efficacy of ACTH therapy against West syndrome, we tried a new 2-steps therapeutic protocol consisting of the shortened ACTH therapy and the additional ACTH therapy. In a prospective multi-institutional study, 20 patients with newly diagnosed West syndrome who had failed to respond to high-dose vitamin B6 and zonisamide were treated by this shortened ACTH therapy. Synthetic corticotropin (ACTH-Z 0.025 mg/kg/dose, max 0.25 mg) was administrated intramuscularly seven times on every other day for 14 days. At 1 month after discontinuing corticotropin, spasms and hypsarrhythmia disappeared in 10/20 (50%) and 13/17 (59%) patients respectively. Subsequently, 9 out of the 10 patients with persistent spasms received additional therapy for 1 or 2 weeks with daily intramuscular ACTH-Z, which was tapered off over a few weeks. Including the additional ACTH therapy, the disappearance of spasms and hypsarrhythmia were found in 13 patients (65%) and 13 patients (76%). Adverse effects during the shortened ACTH therapy were fewer than additional ACTH therapy but not statistically significant. Severe adverse effects were not observed in both ACTH therapy. In the 2-steps therapeutic protocol according to the response to ACTH, favorable results were obtained in seizure control, EEG findings and the degree of adverse effects.