• During an outbreak investigation of necrotizing enterocolitis (NEC) in a neonatal intensive care unit, we identified nine definite and six suspected cases of NEC on the basis of histopathologic, clinical, and roentgenographic findings. Neonates of low birth weight (<1,250 g) had the highest incidence of NEC, supporting a role for prematurity in this disease. Patients with definite NEC and those with severe clinical features had significantly lower birth weights and postconception ages (gestational age at birth plus postnatal age at onset of NEC) than the patients with suspected NEC. In a case-control study using birth weight-matched control subjects, maternal toxemia was identified as a possible protective factor for NEC. To our knowledge, this is the first report of the relationship between NEC disease severity and postconception age. These findings also suggest that toxemia may be an important protective factor in NEC and should be examined in subsequent studies.