In a study of 15 patients with infantile spasms and Lennox syndrome 4 (26.7%) showed no cerebral atrophy and 11 (73.3%) showed mild ventricular and sulcal enlargement before ACTH therapy. Ventricular and sulcal enlargement increased soon after ACTH therapy in most patients. Cerebral atrophy was reversible at over 1 month after ACTH therapy. However, in three instances, cerebral atrophy was not reversible, and one patient with infantile spasms had chronic subdural effusion, which appeared to be a complication of treatment with ACTH. Patients on ACTH therapy should be carefully monitored.