Fifty patients with primary hyperparathyroidism were studied with an oral calcium-tolerance test, measurements of plasma levels of vitamin D metabolites, and determination of calcium excretion on both a low-normal (400 mg) and high-normal (1000 mg) calcium intake. There were strong positive correlations between plasma levels of 1,25-dihydroxyvitamin D (1,25(OH)2D) and both the calciuric response to the calcium-tolerance test (r = +0.75, P less than 0.001) and calcium excretion on the 1000-mg calcium diet (r = +0.65, P less than 0.001). The patients were classified into two subpopulations: 30 patients showed hyperabsorption with the calcium-tolerance test, striking hypercalciuria, marked elevations in plasma 1,25(OH)2D, and a high incidence (19 of 30 patients) of renal stones; 20 patients had a normal response to the tolerance test, normocalciuria, normal or high-normal plasma 1,25(OH)2D, and a low incidence of stones (three of 20 patients). The findings emphasize the importance of circulating 1,25(OH)2D in the pathogenesis of hypercalciuria and stone formation in primary hyperparathyroidism.