Selective catheterization of the inferior petrosal sinus has been performed in 23 patients: 11 Cushing's disease, 5 lung carcinoid tumors whose diagnosis has been confirmed by surgery and 7 ACTH-dependent hypercortisolism whose final diagnosis has not yet been proved. The pituitary origin of hormone secretion is accepted if the central-to-peripheral ratio (C/PR) is equal or greater than 2 on basal values and/or during CRF test. On basal values, all subjects but one with confirmed pituitary lesions exhibited a C/PR > or = 2; the carcinoid tumors never exceeded the value of 1.3. After CRF injection, all the pituitary tumors showed an increase in C/PR; one patient with a lung carcinoid tumor (CRF-negative) showed a pituitary ACTH secretion (C/PR = 3.7). The localization of pituitary adenoma by the lateralization of hormone secretion is disappointing, except, perhaps, for very small extremely lateral adenomas. No accident occurred in our series, nor in the literature. This investigation seems the most reliable approach to prove the pituitary origin of hormonal hypersecretion. We recommend it to be performed in ACTH-dependent Cushing's syndrome when the secretory pattern is not typical and/or pituitary imaging is normal.