Preterm infants fed human milk have been shown to grow poorly and develop mineral deficiencies that may lead to osteopenia. This study has investigated the efficacy of a human milk fortifier made up of glucose polymers, a mixture of bovine milk protein fractions and free amino acids, minerals and vitamins designed to improve these nutritional deficiencies. Growth and bone mineral deficiencies were compared in 38 preterm infants fed fortified mother's milk and 21 preterm infants fed a preterm formula until they reached 1800 g; all had a birthweight below 1600 g. Weight gain was similar in each group with a mean (SD) increase of 19.6 (3.5) g/kg/day in the fortified group and 19.9 (4.1) g/kg/day in the preterm formula group. There were also no significant differences in linear growth, head circumference, skin fold thickness or mid-arm circumference. Serum phosphate, alkaline phosphatase and plasma urea concentrations were similar and there was no clinical evidence of osteopenia. These results indicate that the growth and metabolic disadvantages associated with feeding human milk to preterm infants are ameliorated by the addition of a milk fortifier that increases the calorific, protein and mineral content of breast milk.