In four out of seven patients with primary hyperparathyroidism, we have found elevated plasma renin activity (PRA) and blood pressure, both of which returned to normal following surgical correction of the hyperparathyroidism. However, PRA was normal in normotensive patients with primary hyperparathyroidism, those with hypercalcemia of other etiologies, and those with secondary hyperparathyroidism. These findings suggest that the renin-angiotensin system may play a role in the etiology of the hypertension in primary hyperparathyroidism.