SummarySince the first report in 1928 about 200 tumors with ectopic ACTH syndrome have been reported. From hormonal assays in serum and in tumor tissue of unse-lected cases with carcinoma it can be inferred that this paraneoplastic Cushings' syndrome could exist in about 10 % of visceral carcinomas; it seems therefore that many incomplete or asymptomatic cases are overlooked. A review of clinical and biological features shows that parameters for the diagnosis of ectopic ACTH production are unreliable and that, without a final proof with the demonstration of a ACTH-like substance in the tumor tissue, some cases cannot be distinguished from tumors with pituitary-dependent hypercortisolism.