Despite potential benefits, human milk may fail to meet preterm infants' nutrient requirements. We tested the hypothesis that fortified breast milk, fed alone or with preterm formula, would improve neurodevelopment and growth at 18-mo follow-up without adverse short-term clinical or biochemical consequences. Two hundred seventy-five preterm infants from two medical centers (birth weight < 1850 g; mean gestation 29.8 +/- 2.7 wk) whose mothers chose to provide breast milk were randomly assigned to receive for a mean of 39 d a multinutrient fortifier or control supplement containing phosphate and vitamins. Breast milk comprised 47.6% and 46.4% of enteral intake in fortified and control groups, respectively; preterm formula supplements were used when insufficient breast milk was available. Overall, there were no significant growth advantages with fortification; although, when breast milk exceeded 50% of intake, fortification promoted faster weight gain (an advantage of 1.6 g.kg-1.d-1; 95% CI: 0.1, 3.1; P < 0.05). Compared with control infants, the fortified group showed 1) higher plasma urea from week 2 (P = 0.04), 2) higher plasma calcium (mean 2.34 +/- 0.01 compared with 2.27 +/- 0.02 mmol/L; P = 0.003), 3) a greater rise in alkaline phosphatase by week 6 (P = 0.04), 4) more clinical infections (suspected plus proven; 43% compared with 31%, P = 0.04), 5) a nonsignificantly increased incidence of necrotizing enterocolitis (5.8% compared with 2.2%, P = 0.12), and 6) higher white cell and platelet counts. Developmental scores at 18 mo were slightly but not significantly higher in the fortified group. This study confirmed that breast milk fortifiers can improve short-term growth (when breast milk intakes are high); but beneficial effects on long-term development remained unproven. Future research is required to evaluate potential adverse consequences and explore more optimal fortification strategies.