Ten patients with recurrent renal calculi were evaluated for parathyroid function. Each patient had persistently normal serum calcium values, normal or low serum phosphorus values, and increased urinary excretion of calcium, thus satisfying the criteria for "idiopathic hypercalcuria." However, each patient had reduced tubular resorption of phosphorus (TRP) in the presence of normal renal function, consistent with hyperparathyroidism. Four grossly normal parathyroids were identified in each patient at operation; three or 3½ glands were removed in each instance. Parathyroid hyperfunction was evident by electron microscopy in six of six patients examined. The clinical and biochemical response to parathyroidectomy corroborated the diagnosis of hyperparathyroidism. The incidence of stone formation was dramatically reduced. There was a prompt and sustained fall in urinary calcium, a rise in TRP, and an insignificant fall in serum calcium value. No patient had persistent hypocalcemia despite removal of 3 or 3½ parathyroid glands. In all four patients in whom serum parathyroid hormone levels were determined, the preoperative level was significantly elevated and fell promptly to normal postoperatively.