Whether early diet influences long-term health or achievement is a key question in nutrition. Such long-term consequences would invoke the concept of 'programming'--a more general process whereby a stimulus or insult at a critical period of development has lasting or lifelong significance. Data from small mammals and primates show that early nutrition may have potentially important long-term effects, for example on blood lipids, plasma insulin, obesity, atherosclerosis, behaviour and learning. Corresponding studies in man have been largely retrospective and difficult to interpret. The preterm infant is however an important model for human research because formal random assignment to early diet is practical. A large prospective randomized multicentre study has been undertaken on 926 preterm infants to test the hypothesis that early diet influences long-term outcome. Diets included human milk, standard formula and nutrient-enriched preterm formula. The diet consumed for on average the first month post partum had a major impact on subsequent developmental attainment, growth and allergic status in early childhood. That such a brief period of dietary manipulation has lasting significance implies that the neonatal period is critical for nutrition after preterm birth. These data may have broader implications for human nutrition.