Two patients with Cushing's syndrome are reported. The syndrome was the consequence of relapse after transphenoidal resection of a pituitary tumor in one and due to occult ACTH secretion in the other. Determination of ACTH after bilateral petrosal sampling in both cases and after systemic venous sampling at different sites in one, allowed localization of the source of hormone secretion which was confined to one side of the hypophysis in one case and to the thorax in the other. Hypophyseal resection in the first case and bilateral adrenal resection in the other were performed successfully. The value of this diagnostic test is analyzed.