In 16 patients with hypokalemic hypertension the combination of abnormally high and unsuppressible plasma aldosterone with low or undetectable renin activity led to the diagnosis of primary aldosteronism. To differentiate between aldosterone producing adenoma and idiopathic bilateral hyperplasia, determination of aldosterone concentration in both adrenal veins was performed in 12 patients. In 4 of these patients the two forms of primary aldosteronism could not be differentiated as in these cases only one of the two adrenal veins simultaneously showing an abnormally high aldosterone concentration could be canulated. Plasma aldosterone and plasma cortisol were determined overnight (20.00-8.00 h) at short time intervals in 8 patients with adenoma, 1 patient with carcinoma of the adrenal cortex and 3 patients with bilateral hyperplasis. In all patients with adenoma a significant correlation between aldosterone and cortisol was observed (p less than 0.05-0.001) whereas no correlation was seen in the patients with hyperplasia and carcinoma. The clinical importance of these findings is that in the presence of ACTH-dependent secretion of aldosterone the site of the adenoma can be predicted even when blood from only one adrenal vein is obtained.