In a prospective study of the incidence of primary aldosteronism among patients with "essential" hypertension, this diagnosis could be excluded in 87 of 90 patients by the demonstration of normal aldosterone secretion or lack of suppression of plasma renin activity or both. In contrast, at least four of ten patients observed during the same period because of concomitant hypertension and unprovoked hypokalemia were proven to have primary aldosteronism. Primary aldosteronism is, thus, a relatively rare cause of hypertension among patients with normal serum potassium concentrations, but should be carefully considered as a possible cause of hypertension among patients with hypokalemia. Subnormal plasma renin activity was found to be a characteristic of patients with "essential" hypertension as a group; marked suppression of plasma renin activity was found in five (21%) of 24 patients with "essential" hypertension. Thus, although subnormal plasma renin activity can be used to distinguish primary from secondary aldosteronism, taken by itself, this finding is of limited value in diagnosis since it also occurs in many patients with "essential" hypertension.