The serum concentration of 1,25-dihydroxylvitamin D (1,25-[OH]2D) in normal children and in children with inherited diseases of bone was compared by use of a competitive binding assay. Observed values were: in 12 normal children and adolescents, 37.1 +/- 1.9 pg per milliliter (mean +/- S.D.); in 14 patients with X-linked hypophosphatemic rickets treated with vitamin D2 and phosphate supplements, 15.6 +/- 7.8 (P less than 0.01 versus control); in six patients with autosomal recessive vitamin D dependency treated with vitamin D2, 9.5 +/- 2.9 (P less than 0.01 versus control); and in four untreated patients with autosomal dominant hypophosphatemic (non-rachitic) bone disease, 30.2 +/- 6.3 (not significantly different from the controls). The difference in bone disease between X-linked hypophosphatemia (severe) and hypophosphatemic bone disease (mild) at comparable low serum levels of phosphate implies that 1,25-(OH)2D and phosphate may have independent roles in the pathogenesis of defective bone mineralization.