Ectopic ACTH syndrome is a clinicopathologic condition produced by certain tumors which release hormone that is indistinguishable from ACTH. It orginates the chemical and clinical anomalies characteristic of Cushing's syndrome by its action on the adrenal glands. The tumors may be present in any organ, though they are most frequently found in the lungs, thymus, pancreas or gastrointestinal tract. They may be benign or malignant, though usually the latter. Secretion of the hormone is completely autnomous; it is release in a way similar to that of the hypophysis. Not infrequently other hormones besides ACTH are also produced, such as MSH, serotonin, and CRF. Ectopic ACTH is of higher molecular weight than hypophyseal ACTH, which suggest it may be comprised of the latter bounded covalently to a peptide. The clinical course is rapid, so that not all of the symptoms of Cushing's syndrome develop. Moon face, osteoporosis, and obesity are typically lacking; melanodermia and hypokalemic alkalosis ofter appear. Laboratory data include an increase in ACTH and cholesterol concentrations, disappearance of the nictameral rhythm, and an increase in urinary excretion of 17-hydroxycorticoids and 17-ketosteroids. Stimulation and supression tests are abnormal. The prognosis is poor and the only possible treatment is a complete surgical removal of the tumor. Irradiation or chemotherapy could be applied as well as the correction of the adrenal hyperfunction by the administration of drugs or by total bilateral adrenalectomy.