The syndrome of primary aldosteronism is caused either by an aldosterone-producing adenoma or by idiopathic bilateral adrenal hyperplasia. Hypokalemic hypertension is the leading symptome of the disease. Diagnosis is by the combination of abnormally high and non-suppressible aldosterone values with undetectable or low renin values unresponsive to postural changes or salt restriction. Patients with aldosterone-producing adenoma normally show a fall in plasma aldosterone in response to posture and ACTH-dependent circadian rhythm of aldosterone, whereas bilateral hyperplasia is characterized by postural increases in plasma aldosterone and an ACTH-independent diurnal aldosterone rhythm. These creteria serve to differentiate between adenoma and hyperplasia. An aldosterone-producing adenoma can be localized by veinography, determination of aldosterone concentration in both adrenal veins and by 131I-cholesterol scintigraphy. In our hands the determination of aldosterone in blood from both adrenal veins is the most efficient procedure. In interpreting the results, however, rhythmic and sudden changes in adrenal hormone secretion should be considered. In cases where no adrenal venous blood is obtained, 131I-cholesterol scintigraphy may be used to localize adenoma. In patients with aldosterone-producing adenomas unilateral adrenalectomy should be performed, whereas patients with idiopathic bilateral hyperplasia should receive antihypertensive therapy. As rare instances of primary aldosteronism, a case of aldosterone-producing carcinoma of the adrenal cortex and a case of presumably unilateral adrenal hyperplasia are reported.