Objectives– Phasic inhibition index (PII) is the rate of the simultaneous occurrence of phasic chin muscle activity (PCMA) and rapid eye movement bursts during rapid‐eye‐movement sleep (REMS). In naive patients with infantile spasms (IS), the PII value was found to reflect their prognosis. We studied the effects of adrenocorticotropic hormone (ACTH) on REMS components including PII in IS. Methods– REMS parameters were examined in 18 IS patients before and after ACTH treatment. The effects of corticosteroids (CSs) were examined in 3 patients with congenital adrenal hyperplasia (CAH) and 3 with nephrotic syndrome (NS). Results– ACTH decreased PII and PCMA in IS patients. In CAH patients, physiological doses of CSs corrected the increased intrinsic ACTH level and increased PII. In NS patients, therapeutic doses of CSs suppressed PCMA without affecting PII. Conclusion– ACTH suppressed PCMA through CSs, and reduced PII directly. ACTH was hypothesized to eliminate IS through these dual modes of action.