An 8-month old male with Cushing's disease is presented; his clinical presentation and appearance were typical of infants with glucocorticoid excess. Concentrations of cortisol, 17-hydroxyprogesterone, and adrenal androgens were strikingly elevated. High doses of dexamethasone did not suppress the excretion of urinary free cortisol or 17-hydroxycorticoids, and administration of ACTH elicited no further rise in plasma cortisol. Responses of LH, FSH, and PRL to iv LRF and TRF were appropriate for age, but neither TSH nor ACTH rose significantly. Plasma ACTH values were elevated to 700 pg/ml. An intracranial mass lesion superior and anterior to the sella turcica was demonstrated by computerized axial tomography and angiography. An inoperable pituitary adenoma was a massive surrounding fibroblastic reaction was found at craniotomy. The pathological diagnosis of an ACTH-producing pituitary adenoma was confirmed by immunohistochemistry and by the in vitro secretion of ACTH by cells cultured from the tumor.