Two forms of X-linked hypophosphatemia occur in the mouse. One form, caused by the Hyp gene, is a counterpart of human X-linked hypophosphatemic "vitamin D-resistant rickets". The other, recently characterized, is caused by a different gene (Gy) closely linked to Hyp on the mouse X-chromosome. The Gy mutation also impairs cochlear function in the mouse. We measured hearing in 22 patients with X-linked hypophosphatemia; five, including 2 mother-son pairs, had sensorineural hearing deficits due to cochlear dysfunction. We suggest the disease in these persons may be the human counterpart of the Gy phenotype in the mouse, which implies there are 2 forms of X-linked hypophosphatemia in humans.