This report compares fat, nitrogen, calcium, phosphorus, zinc, and copper absorption and retention data from 13 nutritional balance studies performed in 12 appropriate-for-gestational-age premature infants with birth weights less than or equal to 1,600 g fed a proprietary premature formula or their own mother's preterm human milk (PTHM) fortified with a powdered protein-mineral supplement. At the time of each balance study, each infant had a stable condition, was tolerating feedings, and was gaining weight steadily. Stool and urine were collected separately; doses of carmine red given 72 h apart were used to define dietary intake and the stool and urine collections. The balance studies were performed at an average age of 36 postnatal days. Both diets were found to support weight gain and nutrient retention at similar rates. Balance studies (n = 7) with premature formula demonstrated a weight gain of 16.8 +/- 5.2 g/kg/day, with a net fat absorption of 6.36 +/- 0.97 g/kg/day, net nitrogen retention of 427.9 +/- 47.1 mg/kg/day, net calcium retention of 95.0 +/- 14.1 mg/kg/day, net phosphorus retention of 56.7 +/- 5.9 mg/kg/day, net zinc retention of 208 +/- 903 micrograms/kg/day, and net copper retention of 26.4 +/- 30.8 micrograms/kg/day (mean +/- SD). Comparison balance studies (n = 6) with fortified PTHM demonstrated a weight gain of 17.2 +/- 7.1 g/kg/day, net fat absorption of 5.60 +/- 1.55 g/kg/day, net nitrogen retention of 366.0 +/- 84.0 mg/kg/day, net calcium retention of 82.2 +/- 10.8 mg/kg/day, net phosphorus retention of 58.6 +/- 5.9 mg/kg/day, net zinc retention of 685 +/- 363 micrograms/kg/day, and net copper retention of 67.3 +/- 37.2 micrograms/kg/day. These rates of weight gain and of fat, nitrogen, zinc, and copper retention approximate those of the third trimester of intrauterine life; only calcium and phosphorus retention approached the lower range of estimated in utero accretion.