Abnormal renal tubular phosphate transport is considered to be the primary defect in X-linked hypophosphatemic rickets (XLH). However, the resistance to vitamin D treatment in XLH cannot be explained by hypophosphatemia alone. Since most of the actions of vitamin D are mediated by its receptors (VDR), abnormalities of VDR have been postulated in XLH. In order to investigate this possibility, we measured the concentration of VDR in PHA-activated peripheral mononuclear cells from 10 XLH patients. Patients without phosphate supplementation showed significantly lower concentration (21.7 +/- 5.1 fmol/mg protein, mean +/- SEM) compared to the normal controls (60.7 +/- 4.0). On the contrary, there was no significant difference between the phosphate-supplemented patients (58.3 +/- 2.7) and controls. There was a significant positive correlation between VDR concentration and serum phosphate (P less than 0.05). In two patients, VDR was increased after daily phosphate supplementation was started. These results indicate that a decreased concentration of VDR secondary to persistent hypophosphatemia is one of the causes of vitamin D resistance in XLH.