The medical records of 32 patients with primary aldosteronism who underwent adrenalectomy at the University of Michigan Medical Center from January 1975 to February 1988 were reviewed. All 32 patients had the preoperative diagnosis of aldosterone-secreting adrenal cortical neoplasms. Based on pathology reports, however, 21 of 32 (66%) patients were confirmed to have adrenal cortical neoplasms. Ten of 32 (31%) patients had nodular hyperplasia, and 1 of 32 (3%) had diffuse hyperplasia. This report focuses on the results in 11 patients with idiopathic aldosteronism. In six of nine (67%) patients, aldosterone levels rose within 4 hours of patients assuming an upright posture after salt loading. Seven patients had selective adrenal venous aldosterone/cortisol ratios that were interpreted to lateralize to one adrenal gland; however, only four of seven (57%) had ratios of 3:1 or greater than the contralateral adrenal gland. In 6 of 11 (55%) patients, adrenal scans (NP-59) initially demonstrated unilateral uptake. Three of four computerized axial tomographic scans demonstrated a unilateral adrenal mass. Only 3 of 11 (27%) patients with idiopathic aldosteronism were normotensive after surgery. Four of 11 (36%) patients' conditions were improved, in that they became normotensive with antihypertensive medication. These data suggest that if both imaging and functional studies lateralize to one adrenal gland, it is reasonable to expect either a cure or an improvement after adrenalectomy among patients with primary aldosteronism caused by idiopathic aldosteronism. Unilateral adrenalectomy may be the treatment of choice in carefully selected patients with nodular hyperplasia causing primary aldosteronism.