In order to determine whether a defect in vitamin D metabolism might play a role in the pathogenesis of X-linked hypophosphatemic rickets and osteomalacia (XLH), we compared the serum 1,25-dihydroxyvitamin D [1,25(OH)2D] level in 52 normal subjects and 37 patients with XLH. In untreated patients, adults were found to have values similar to age-matched controls, while youths had values similar to growth-rate-matched controls but significantly lower than the levels of age-matched controls who were growing at a normal rate. In contrast, treated XLH patients of all ages had serum levels significantly lower than both controls and untreated XLH patients. Further, the serum levels of 1,25(OH)2D in these treated patients had a significant inverse linear correlation with serum 25-(OH)D concentrations. We propose that subjects with XLH have serum 1,25(OH)2D levels within appropriate age- and growth-rate-matched normal ranges. However, in the presence of hypophosphatemia, we would have anticipated elevated levels of 1,25(OH)2D; viewed in this light the serum 1,25(OH)2D levels are inadequate, suggesting the presence of a relative deficiency of this active vitamin D metabolite.