An outpatient diagnostic procedure measuring the 6-hour integrated plasma concentration of aldosterone and plasma renin activity was used to detect primary aldosteronism in 12 patients with low renin hypertension, including six with mild hypertension and normal urinary excretion and spot plasma levels of aldosterone. The ratio of integrated plasma concentration of aldosterone to plasma renin activity in the 12 patients (mean, 339; range, 116–700;p< 0.0001) did not overlap with that measured in 105 normotensive controls (mean, 27.8; range, 5–97) or in 87 subjects with essential hypertension (mean, 29.2; range, 4–67). Eight patients had surgically proven adenomas (3 of which measured <5 mm) with normalization of blood pressure following adrenalectomy. The four remaining patients had bilateral hyperplasia. The 6-hour integrated plasma concentration of aldosterone to plasma renin activity ratio was found to be a useful new outpatient diagnostic tool for evaluation of primary hyperaldosteronism.