Adrenal venous aldosterone/cortisol ratios were determined in 14 patients with primary aldosteronism. Of 12 patients diagnosed as having an adrenal adenoma, ten underwent surgery in which an adenoma was found. Bilateral adrenal hyperplasia was thought to have developed in two other patients. Measurement of adrenal vein cortisol and establishment of aldosterone/cortisol ratios were done to correct for catheter placement and dilute adrenal efflux. Determination of steroids after ACTH stimulation was helpful in distinguishing quiescent from suppressed adrenal in two patients and in confirming the diagnosis of the others.