In 72 patients with primary aldosteronism who were classified on the basis of adrenal pathology after adrenalectomy, analysis of routine clinical and laboratory data, of supine and upright plasma aldosterone, and of plasma renin activity were of limited value in differentiating patients with aldosterone-producing adenoma(s) (APA, n = 59) from those with idiopathic adrenal hyperplasia (IAH, n = 13). Normokalemic aldosteronism occurred in 6 patients (3 APA, 3 IAH). A correct classification of the adrenal lesion(s) was obtained in 80% of the patients by computed tomography and only in 69% by adrenal scintiscan. In addition, adrenal scintiscan was hampered by a relatively high rate of incorrect results independent of whether dexamethasone was used or not. Small adenomas ( < 1 cm) and more often adrenal hyperplasia may escape visualization by computed tomography.