Over a 7 year study period, 82 infants were identified who had necrotising enterocolitis (NEC). A case-control study of the 74 preterm infants was performed to determine those factors which contributed to the development of NEC. The 35 infants with NEC and gestation between 30-36 weeks, when compared with control infants matched for gestational age, had significantly lower birthweight centiles, cord pH, and 1 minute Apgar scores. By contrast, there were no significant differences between the 39 infants with NEC and controls in the 25-29 week group, except that fewer babies with NEC had received breast milk. The eight term babies all appeared to have an obvious predisposing event. We thus propose a model in which susceptibility to NEC is dependent on gestational age. In the 25-29 week range all babies are at risk on the basis of extreme prematurity. In the 30-36 week range asphyxiated and growth retarded babies are at increased risk, while at term a major predisposing event appears to be required.