The apparent vitamin B-6 status of 31 children with familial hypophosphatemic rickets (FHR) was determined. All children had alkaline phosphatase activity that was high-normal to elevated for their ages. A sensitive assay for pyridoxal 5'-phosphate (PLP) indicated that 15 of the 31 children had an undetectable (less than 0.2 nmol/L) concentration of the vitamer--the lowest values yet reported in human serum. The 16 remaining children had concentrations of the vitamer so low that they indicated a potential severe vitamin B-6 deficiency. However, none of the children had ever presented with any of the classical vitamin B-6-deficiency symptoms. Treatment of three additional FHR children with 100 mg pyridoxine.HCl/d resulted in a moderate and transient elevation of their serum PLP concentrations, a dramatic elevation of their erythrocyte PLP concentrations, and no improvement in clinical condition. Serum or plasma PLP concentrations are an inappropriate index for determining vitamin B-6 status in people with FHR and perhaps in others with elevated alkaline phosphatase activity.