The influence of severe hypophosphatemia (less than or equal to 1.0 mg/dl) on vitamin D metabolism was prospectively determined in 11 patients before and after intravenous phosphorus administration. Evidence of liver dysfunction was present in ten patients. The mean (+/- SE) plasma 25 hydroxycholecalciferol [25(OH)D] was significantly decreased before phosphorus therapy when compared to control subjects (9.4 +/- 1.3 vs. 17.8 +/- 1.3 ng/ml, P less than 0.001). With phosphorus administration, serum phosphorus increased from 0.59 +/- 0.07 to 2.58 +/- 0.09 mg/dl while 1,25 dihydroxycholecalciferol [1,25(OH)2D] decreased from 34.6 +/- 4.3 to 14.3 +/- 2.9 pg/ml (P less than 0.001). Plasma 25(OH)D, plasma immunoreactive PTH (both amino and carboxyterminal) and serum calcium did not change after phosphorus administration, suggesting that phosphorus alone was responsible for the change in plasma 1,25(OH)2D concentration. An inverse correlation was found between serum phosphorus and plasma 1,25(OH)2D (r = -0.62, P less than 0.005). In addition, a direct correlation was observed between plasma 25(OH)D and 1,25(OH)2D both before (r = 0.66, P less than 0.005) and after (r = 0.74, P less than 0.005) phosphorus administration. Thus, the decrease in 1,25(OH)2D levels with phosphorus therapy suggests a role of serum phosphate in the regulation of this sterol, and hypophosphatemia or phosphorus depletion may change the relationship of substrate [25(OH)D] to product [1,25(OH)2D].