In a prospective study of 80 patients with primary aldosteronism (70 with adenoma and 10 with hyperplasia), "refractory" hypertension, hyperkinetic circulation, and hypovolemia were frequent occurrences. We found that measurements of serum potassium concentration and plasma renin activity were inadequate screening tests because of high rates of false-positive and false-negative results. The demonstration of excessive aldosterone production after three days of salt loading provided the best sensitivity (96 percent) and specificity (93 percent) in identifying patients with primary aldosteronism. Severe, persistent hypokalemia, increased plasma 18-hydroxycorticosterone values, and an anomalous postural decrease in the plasma aldosterone concentration, when present, provided the best indicators of the presence of an adenoma. Of three localizing procedures (selective adrenal venography, adrenal computed tomographic scan, and adrenal venous sampling for plasma aldosterone concentration) the measurement of adrenal venous plasma aldosterone concentration yielded 100 percent accuracy. These results indicate a wider clinical spectrum in primary aldosteronism than previously described. They also show that nonsuppressible aldosterone production is its most important diagnostic hallmark and the single best diagnostic screening procedure, and that adrenal venous sampling for plasma aldosterone concentration remains the most precise technique for identification and localization of tumors.