X-linked hypophosphatemic rickets (HYP) is an X-linked dominant disorder characterized by decreased renal tubular phosphate reabsorption and consequent hypophosphatemia. Renal cross-transplantation studies in Hyp mice indicate that the disorder is secondary to the elaboration of an as yet unidentified humoral factor. A full understanding of the pathophysiology of the disease and the nature of this factor will be facilitated by identification of the HYP gene. Efforts to isolate the HYP gene have been deterred by limited precision in the map of the Xp22.1 region and the consequent distance between DXS365 and DXS274, the previously discovered flanking markers for the HYP gene. To map the HYP region precisely, HYP family resources from two groups of investigators were combined, and several newly available microsatellite repeat probes were tested for linkage to HYP. Our data indicate that DXS365, DXS3424, DXS443, DXS1052, DXS274, and DXS1683 are tightly linked to the HYP gene and suggest a locus order of: Xtel-DXS315-(GLR/DXS43)-DXS257-(DXS443+ ++-DXS3424)-DXS365-HYP-DXS1683-DXS1052-DXS 274-(DXS41/DXS92)-DXS451-Xcen. The HYP gene is located in the 350- to 650-kilobase region between DXS365 and DXS1683. These results will provide a basis for the isolation of candidate genes from the region.