Isolated ACTH deficiency (IAD) is a rare disorder, characterized by secondary adrenal insufficiency (AI) with low or absent cortisol production, normal secretion of pituitary hormones other than ACTH and the absence of structural pituitary defects. In adults, IAD may appear after a traumatic injury or a lymphocytic hypophysitis, the latter possibly due to autoimmune etiology. Conversely, a genetic origin may come into play in neonatal or childhood IAD. Patients with IAD usually fare relatively well during unstressed periods until intervening events spark off an acute adrenal crisis presenting with non specific symptoms, such as asthenia, anorexia, unintentional weight loss and tendency towards hypoglycemia. Blood chemistry may reveal mild hypoglycemia, hyponatremia and normal-high potassium levels, mild anemia, lymphocytosis and eosinophilia. Morning serum cortisol below 3 microg/dl are virtually diagnostic for adrenal insufficiency. whereas cortisol values comprised between 5-18 microg/dl require additional investigations: insulin tolerance test (ITT) is considered the gold standard but-when contraindicated-high or low dose-ACTH stimulation test with serum cortisol determination provides a viable alternative. Plasma ACTH concentration and prolonged ACTH infusion test are useful in differential diagnosis between primary and secondary adrenal insufficiency. For some patients with mild, near-to-asymptomatic disease, glucocorticoid replacement therapy may not be required except during stressful events; for symptomatic patients, replacement doses i.e., mean daily dose 20 mg (0.30 mg/kg) hydrocortisone or 25 mg (0.35 mg/kg) cortisone acetate, are usually sufficient. Administration of mineralocorticoids is generally not necessary as their production is maintained.