West syndrome is a rare epileptic disease of infancy, typified by an association of characteristic spasms, hypsarrhythmia on electroencephalography and severe psychomotor retardation or deterioration. Adrenocorticotropic hormone (ACTH) is the current first-line therapy for West syndrome despite the fact that ACTH therapy is associated with various adverse effects. We describe a rare case of iatrogenic diabetes mellitus during ACTH therapy in a patient with symptomatic West syndrome. The infant had cushingoid facies, hirsutism and biochemical evidence of diabetes due to excessive glucocorticoid production with hyperplasia of both adrenal glands at ultrasound examination, without mineralocorticoid excess; in addition, he presented also short-term weight gain, marked electrolyte disturbances, hypokalemic alkalosis and infections. When ACTH is used to treat patients with West syndrome, it is necessary to follow glycemic levels until to the end of therapy.