To evaluate the effect of varying phosphorus intake with constant high calcium intake (430 micrograms/kJ, or 180 mg/100 kcal), we randomly assigned 35 appropriate-for-gestational-age healthy male infants (birth weight, 715-1510 g) on day 21 postnatally to either standard-phosphorus (215 micrograms/kJ, or 90 mg/100 kcal), moderate-phosphorus (254 micrograms/kJ, or 106 mg/100 kcal), or high-phosphorus intake (287 micrograms/kJ, or 120 mg/100 kcal). Three-day mineral balances were determined after 7 d of the study diets. Weight and head circumference gain and intake of energy and vitamin D were not different for all groups. Calcium retentions were sufficient to meet intrauterine accretion in all groups. Phosphorus calculated to be available for soft tissue was significantly higher in the moderate- and high-phosphorus groups, and was sufficient to support soft tissue phosphorus accretion in these two groups. Total absorbed phosphorus and phosphorus tubular reabsorption were each affected by phosphorus intake. We conclude that very low birth weight infants fed high calcium may require greater phosphorus intake than that provided by formulas containing a ratio of calcium to phosphorus of 2:1. We suggest that the optimal mass ratio of calcium to phosphorus for formula for very low birth weight infants is from 1.6:1 to 1.8:1.