Human milk is often inadequate nutritionally for preterm infants. We investigated the effect of adding a commercially prepared milk fortifier to human (maternal or bank) milk and measured changes in lower leg length velocity (LLLvel) using knemometry, weight gain and biochemical indices of nutrition. Babies were allocated to one of three feed groups, in a semi-randomized fashion, to receive human milk alone (group I), fortified human milk (group II) or a preterm formula (group III). The birthweights (median and R) and birth gestations (median and R) of the three groups were as follows: group I 1099 g (654-1248 g) and 28 wk (26-32 wk); group II 838 g (742-1340g) and 31 wk (28-36); group III 1136g (624-1552g) and 32 wk (27-36 wk). All babies who received fortified milk either showed significant (p = 0.0004) acceleration in LLLvel during the period studied, or maintained their pre-study period velocity. This increase in LLLvel was comparable to that achieved by a group of babies given a standard preterm infant formula (p < 0.001). By comparison, the control group's change in LLLvel was more modest (p = 0.04). Babies who received human milk with the fortifier added had the lowest serum levels of alkaline phosphatase at the end of the study period when compared to the other two groups. Other biochemical indices were similar in the three feed groups. No adverse clinical events were encountered which could be attributed to the use of the breast milk fortifier.