Seven siblings with hyperphosphatemic tumoral calcinosis were studied using metabolic measures. Serum phosphorus and 1, 25-dihydroxycholecalciferol concentrations were significantly increased and serum parathyroid hormone and 25-hydroxycholecalciferol concentrations were significantly decreased in these subjects. Metabolic balance studies done in three of the siblings showed positive calcium and phosphorus balances, reflected by increased gastrointestinal absorption and decreased renal excretion. These data suggest that a hereditary abnormality of vitamin D metabolism may be present in patients with hyperphosphatemic tumoral calcinosis. Failure of the normal feedback mechanism regulating the 25-hydroxy-1-alpha-hydroxylase enzyme is suggested as the major cause. Although this defect could lead to many of the metabolic abnormalities seen in these patients, the overall contribution of altered vitamin D metabolism to the pathogenesis of tumoral calcinosis is not fully understood.