A controlled metabolic study to examine the effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) treatment on the renal handling of phosphate was conducted in nine patients with X-linked dominant hypophosphatemic rickets, including one with autonomous secondary hyperparathyroidism. Administration of 1,25(OH)2D3 resulted in uniform reduction in serum PTH from 63.6 +/- 14.7 (SD) to 49.3 +/- 14.8 muleq/ml (P less than 0.01), elevation of the tubular threshold for phosphate (TmP/GFR) from 1.41 +/- 0.30 to 1.90 +/- 0.31 mg/dl (P less than 0.01) and increase in serum phosphate from 2.6 +/- 0.7 to 3.4 +/- 1.1 mg/dl (P less than 0.01) in eight PTH-suppressible patients. Four patients treated with phosphate before and during the study (group A) excreted significantly more phosphate than those not treated with phosphate (group B) (P less than 0.001). In the control period, group A also had depressed TmP/GFR and higher concentrations of serum phosphate and PTH. With 1,25(OH)2D3 treatment, serum phosphate in group A became remarkably higher than in group B, 4.28 +/- 0.99 vs. 2.55 +/- 0.31 mg/dl (P less than 0.02), whereas serum PTH and TmP/GFR were similar in both groups. A good inverse linear correlation was found between mean serum PTH and mean TmP/GFR of the groups before and after treatment (r = 0.947); whereas, no correlation was found between TmP/GFR and serum calcium. The patient with autonomous secondary hyperparathyroidism, who was also treated with phosphate, had the lowest TmP/GFR. Administration of 1,25(OH)2D3 had no effect on the serum PTH and phosphate concentrations or on TmP/GFR. We conclude that in patients with X-linked dominant hypophosphatemic rickets PTH modulates to some extent the tubular handling of phosphate, and that the importance of this mechanism increases with therapeutic phosphate supplementation. Simultaneous administration of 1,25(OH)2D3 suppressed PTH activity, raised serum phosphate concentrations, and elevated TmP/GFR.