ABSTRACT A 40 year old patient with medullary thyroid carcinoma subsequently developed Cushing's syndrome with presumably ectopic ACTH formation. The immunoreactive calcitonin in the plasma was greatly and ACTH slightly increased. Despite the excessively high calcitonin concentration the patient finally developed a serious osteoporosis which presumably resulted from Cushing's syndrome. In this case at least, a raised endogenous plasma calcitonin level did not prevent an osteoporosis caused by hypercortisolism. The autopsy revealed a bilateral hyperplasia of the adrenal cortex and a normal pituitary gland suggesting that Cushing's syndrome was probably caused by an ectopic ACTH source. Twenty-two additional cases of medullary thyroid carcinoma with Cusing's syndrome reported in the literature have been reviewed. Eighteen out of 23 patients were alkalotic which is typical of an ectopic ACTH production. In these patients the characteristic clinical signs of hypercortisolism were nearly as frequently observed (70 %) as in patients with non-ectopic Cushing's syndrome (91 %). An ectopic Cushing's syndrome occurs only in 2–4 % of advanced cases of medullary thyroid carcinoma and is a serious complication. Among the 23 patients 18 died with an average survival time of only 4.5 months after the diagnosis of Cushing's syndrome was established.