A 36-year-old patient developed marked pigmentation, marked myopathy and severe hypokalaemic alkalosis which at first pointed towards an ectopic ACTH syndrome. The dexamethasone test at a high dose indicated cortisol suppression. A mediastinal tumour was seen radiologically, but the sella was of normal size. Computed tomography provided indirect signs of a sellar space-occupying lesion which suggested an ectopic production of corticotropin-releasing factor (CRF) as cause of the Cushing's syndrome. CRF concentration in antecubital venous blood was markedly elevated to 280 ng/l. The mediastinal tumour was excised and proved to be a carcinoid histologically. Postoperatively the CRF concentration fell to 70 ng/l. An extract of the carcinoid contained 15.5 ng/g wet-weight of CRF and 254 ng/g wet-weight of beta-endorphin. The patient died 5 weeks postoperatively of sepsis with bilateral pneumonia. At autopsy the hypophysis was of normal size but showed nodular ACTH-cell hyperplasia. This was thus a case of Cushing's syndrome resulting from ectopic CRF production in a mediastinal carcinoid tumour.