• Within a 28-day period, necrotizing enterocolitis (NEC) developed In 20 of 38 infants (53%). Patients with NEC were compared with the remaining 18 infants hospitalized at the same time who did not acquire the disease. Complications of pregnancy and labor-delivery and infant care practices did not differ between groups. Mean chronologic age was significantly different between patients with NEC and those without, 29 days v 77 days. Mean postconceptional age at the time of the outbreak was also significantly different, 33.4 weeks v 42.3 weeks. None of the cultures demonstrated a specific common pathogen. The low mortality (5%) and the large number of Infants affected suggest an atypical outbreak of NEC. We could not isolate a causative agent despite extensive epidemiologic investigation, and suggest that postconceptional age delineates those at risk.