The clinical course of 54 patients with necrotizing enterocolitis has been compared to a matched group of 98 control patients. On the day of life that NEC occurred, all 54 NEC patients and 63% of controls were receiving standard formula feedings, both at 80/cal/kg/day. Stool cultures at the time revealed a significantly increased frequency of Klebsiella in NEC as compared to control patients. Our data suggest that the combined presence of certain intestinal bacteria and enteric feedings, perhaps requiring a background of mucosal ischemia, may be of etiologic significance in the development of NEC and its radiologic hallmark, pneumatosis intestinalis.