424 preterm infants were randomly assigned a standard "term" formula or a nutrient-enriched "preterm" formula as sole diets (trial A) or as supplements to mother's own expressed milk (trial B) for a median of 4 weeks postnatally. 18 months post term, blind evaluation of 377 survivors showed that those previously fed preterm rather than term formula had major developmental advantages, more so in motor than mental function; the advantages, in both mental and motor scores, were especially striking in small-for-gestational-age infants and in males. For motor development index in trial A, this advantage was 15 points; in infants born small for gestation, it was 23 points (nearly 1.5 SD). Moderate developmental impairment (developmental index less than 85), notably motor impairment, was considerably more common in the group fed term formula. Infants fed preterm formula also had a small benefit in social maturity quotient. Thus, a short period of early dietary manipulation in preterm infants had major consequences for later development, which suggests that the first weeks may be critical for nutrition.