In hyperphosphatemic tumoral calcinosis, plasma 1,25-dihydroxyvitamin D [1,25(OH)2D] levels are inappropriately elevated, suggesting an abnormality in vitamin D metabolism. To define this abnormality further, we measured vitamin D metabolites in two patients and four controls before and after phosphate depletion. The patients showed elevated plasma levels of 1,25(OH)2D in the basal state. Phosphate depletion reduced serum phosphate in patients from a mean of 6.1 to 2.6 mg/dl; this was accompanied by a rise in plasma 25-hydroxyvitamin D from 33.6 to 41.9 ng/dl, and in 1,25(OH)2D from 67.7 to 93.2 pg/ml. The absolute rise in 1,25(OH)2D was similar to that of controls. EDTA infusion produced a normal increase of serum immunoreactive PTH levels and urinary cAMP excretion. In this form of tumoral calcinosis, 1,25(OH)2D levels are elevated despite hyperphosphatemia, normal immunoreactive PTH, and normal serum calcium concentrations, suggesting an abnormality in the regulation of 1,25(OH)2D synthesis or metabolism, or alternatively, another undefined stimulus for 1,25(OH)2D synthesis. These patients appear to have concurrent abnormalities of renal tubular phosphate transport and vitamin D metabolism.