Patients with X-linked hypophosphatemic rickets exhibit clinically evident derangements that include bowed legs and short stature. Although contemporary treatment may result in healing of the rachitic/osteomalacic disorder and straightening of the lower extremities, therapy often does not stimulate growth. Whether such persistent short stature is related to the variable physical manifestations of the disease, the baseline biochemistries, and/or the biochemical response to treatment remains unknown. Therefore, we studied 12 children with X-linked hypophosphatemia to determine if their growth response to calcitriol/phosphorus therapy was dependent upon anthropomorphic characteristics and/or the pre- and posttreatment biochemistries. We observed that growth responsive and resistant youths exhibited similar serum calcium, phosphorus, and creatinine levels at presentation and during therapy. In addition, sexual development was indistinguishable in both groups and growth kinetics appeared independent of physical deformity. In contrast, growth resistant youths presented at less than the 5th percentile whereas growth responsive children were at the 15th percentile or greater. Thus, our data indicate that growth response to calcitriol/phosphate therapy is not a consequence of the biochemical response to therapy or physical deformities. Rather, the criterion that best predicts the growth response is the height percentile at the inception of therapy.