28 adult patients with radiological evidence of medullary sponge kidney (MSK) were studied. Hypercalcemia and increased serum parathyroid hormone (PTH) values were found in 10 patients (36%). In 7 of them, parathyroid surgery was performed: a single adenoma was found in 6 cases and multiple-gland hyperplasia in 1 case. After surgery, 3 patients had normalization of calcium metabolism; 4 patients had persistence of hypercalciuria with progressive increase in serum PTH values (and recurrence of the adenoma in 1 case). Of the remaining patients, 10 (36%) had definite or marginal hypercalciuria, resulting from renal calcium leak in 8 and from intestinal calcium hyperabsorption in 2 of them. In 8 patients (28%), no evidence of disordered calcium metabolism was found. The association of MSK and hyperparathyroidism is not a chance occurrence. MSK might be a renal anatomical complication of primary hyperparathyroidism, or it might be regarded as an anatomic substrate--or rather as a consequence--of prolonged hypercalciuria, regardless of its pathogenesis. The lack of disordered calcium metabolism in a considerable number of patients, however, shows that the enigma of MSK is still far from being solved.