In the summer of 1999 a cluster of 18 cases of necrotizing enterocolitis (NEC) occurred in a University Hospital in Rome, Italy. The cases presented with mild clinical and radiological signs, and none died. Seventy-two per cent had a birth weight of >2500 g, 66·7% had a gestational age of >37 weeks, 30% presented with respiratory diseases and/or hypoglycaemia. All cases occurred within 10 days of birth and between 5 and 7 days after two clusters of diarrhoea (14 cases). The NEC outbreak had two phases; most cases in the first phase occurred in the at-risk unit, whereas those in the second phase occurred in the full-term unit. In the multivariate analysis, invasive therapeutic procedures, pathological conditions and formula feeding were associated with NEC. Although no predominant common bacteria were isolated, we suggest an infective origin of this outbreak.