Bilateral catheterization of sinus petrosus inferior was performed in ten patients with Cushing's syndrome and five patients with acromegaly. This was done in order to secure the diagnosis (Cushing's syndrome) and localize an adenoma (Cushing's syndrome and acromegaly). Blood from sinus petrosus inferior on both sides and from the peripheral vein was analyzed for concentration of ACTH or growth hormone after injection of ACTH-releasing hormone and growth hormone-releasing hormone respectively. The quotient between the ACTH concentration in blood from sinus petrosus and peripheral blood determines the diagnosis in Cushing's syndrome and the quotient between the concentration of ACTH or growth hormone between right and left sinus petrosus determines localization of the pituitary adenoma. Correct diagnosis and localization of ACTH-producing pituitary adenoma was found in eight of ten patients with Cushing's syndrome. One result was doubtful and one was wrong. In four patients with Cushing's disease (ACTH-producing adenoma) magnetic tomography (MT) was negative, but catheterization showed the presence of adenoma. In acromegalic patients, correct localization of the adenoma was determined in two out of five patients. In the other three patients, anatomical difficulties prevented satisfactory interpretation.