Current treatment of X-linked hypophosphatemia (XLH) employs the combined administration of oral phosphate and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. Although this drug regimen significantly improves the clinical course of the disease in children, the value of medical treatment in symptomatic adults with XLH has not been established. We, therefore, investigated the clinical, biochemical, and histological responses to phosphate and 1,25-(OH)2D3 in 16 symptomatic adult patients with XLH followed for a mean of 4.2 yr. Eighty-seven percent of the patients had an improvement in bone or joint pain with therapy. There was a significant increase in mean serum phosphate (from 0.61 +/- 0.03 to 0.77 +/- 0.03 mmol/L) and urinary calcium excretion (from 2.45 +/- 0.38 to 4.39 +/- 0.44 mmol/day) with treatment. Pretreatment bone biopsies demonstrated findings characteristic of osteomalacia, including abnormally increased osteoid volume and decreased mineral apposition rates. Treatment was accompanied by a significant decrease in osteoid thickness as well as a reduction in mean osteoid volume. However, therapy did not completely normalize these parameters. Disease severity, as assessed by histomorphometric parameters, did not correlate with any pretreatment serum or urinary biochemical measurement, but did seem to correlate with symptom score. Although most patients tolerated therapy without difficulty, 1 patient developed tertiary hyperparathyroidism during treatment and renal insufficiency that progressed despite cessation of therapy. This study provides evidence that therapy with oral phosphate and 1,25-(OH)2D3 in symptomatic adults with XLH can result in significant clinical and histomorphometric improvement.