Locally recurrent, poorly differentiated carcinoma of the prostate was associated with hypokalemic alkalosis, marked hypernatremia, diabetes mellitus of recent onset, and hyperosmolar syndrome. These findings, with mild hypertension, in the absence of clinical features of Cushing's syndrome, suggested an ectopic ACTH syndrome. Plasma ACTH and cortisol levels were markedly elevated, and failed to suppress in response to either low or high-dose dexamethazone administration. The patient's condition deteriorated rapidly. Autopsy findings included carcinoma extensively infiltrating the prostate with extension to the urinary bladder, and metastases confined to the pelvic nodes and soft tissues. The adrenal glands weighed 23 g and showed diffuse hyperplasia. Extract of the prostatic tumor was analyzed for ACTH and showed approximately 40 times normal plasma levels (or about 4,010 pg/g of tissue); ultrastructural features showed secretory granules consistent with ACTH content of the tumor cells. Such cells were positive when stained for ACTH by peroxidase-tagged immunochemical methods. The case fulfills all established criteria for relating excess corticosteroid production and nonpituitary tumors.