ACTH therapy in infantile spasms: relationship between dose of ACTH and initial effect or long-term prognosis. The relationship between the dose of ACTH and the initial effect was investigated in 41 children with infantile spasms. More than 0.015 mg (0.6 IU)/kg/day of ACTH was needed for a good initial response of seizures and electroencephalographic abnormalities. The relationship between the dose of ACTH and long-term prognosis was investigated in 29 patients. There was no relationship between the daily or total ACTH dosage, provided the dose was greater than 0.015 mg (0.6 IU)/kg/day, and the outcome of seizures and electroencephalographic abnormalities; however, ACTH 0.04-0.06 mg (1.6-2.4 IU)/kg/day and a total ACTH dose of 1.1-1.5 mg (44-60 IU)/kg resulted in better mental development than smaller doses of ACTH. Side effects of ACTH increased with dosage. Too small or too large a dose of ACTH does not lead to better mental development. The proper dose of ACTH should be used with careful attention to potential side effects. 