Amiloride is a potassium-sparing diuretic used in spontaneous and diuretic-induced hypokalemia. The effect of amiloride was studied prospectively in 12 patients with primary hyperaldosteronism. Four patients had unilateral adrenal adenomas and eight had bilateral adrenal hyperplasia. All patients were hypertensive and their mean plasma potassium levels were low. Amiloride, 10 to 40 mg daily, was given for 6 mo. Mean plasma potassium levels rose (0.96 mEq/l, P less than 0.001) and remained normal throughout the study without potassium supplementation. Mean blood pressure was lowered by amiloride (22/10 mm Hg, P less than 0.001) but normotension required concomitant antihypertensive therapy in most patients. No significant adverse clinical or laboratory experiences could be directly attributed to amiloride therapy. There was no correlation between the response to therapy and the plasma aldosterone levels, aldosterone secretion rate, or presence of a unilateral adrenal adenoma. Our study demonstrates the efficacy of amiloride in the correction of hypokalemia and amelioration of hypertension in primary hyperaldosteronism.