The combination of aldosterone overproduction, subnormal plasma renin activity, and normal urinary 17-hydroxycorticosteroids is diagnostic of primary aldosteronism, whether hypokalemia is present or absent. Congenital aldosteronism may be a rare exception. Of 68 surgically proven cases these diagnostic criteria were fully satisfied in 64, and almost so in four. In the last 19 patients whose diagnosis was clear by these criteria, adrenal phlebography was added for tumor localization. To date, 13 patients have undergone surgery and all had tumors (12 solitary). Phlebography visualized the tumor in nine of 12 tumorous glands successfully injected. Of six patients awaiting surgery, four manifest phlebographic evidence of tumor. Adrenal phlebography is important for tumor localization and for further research on minimal diagnostic criteria in primary aldosteronism.