• Five affected members in two branches of a Druze Arab kindred presented with calcified deposits in or about the hips and knees, indicating the diagnosis of tumoral calcinosis. Serum vitamin D metabolite concentrations, immunoreactive parathyroid hormone, and the tubular reabsorption of phosphate were measured in three affected siblings, as well as in their parents and an unaffected sibling. Normocalcemia and hyperphosphatemia with an elevated tubular maximum for phosphate per glomerular filtration rate were found in the affected patients. Levels of serum immunoreactive parathyroid hormone were normal, and the calcitriol value (1,25-dihydroxyvitamin D) was normal or elevated in the three affected patients. A trial of a low-phosphorus diet and oral aluminum hydroxide gel did not lower serum phosphate levels or improve the calcified deposits. These findings suggest that serum calcitriol values do not decline in response to hyperphosphatemia in tumoral calcinosis and that hyperphosphatemia with elevated tubular reabsorption of phosphate is a constant feature of this disorder because the kidney fails to excrete the increased filtered load of phosphate.