Detailed attempts to isolate pathogens were made during an apparent epidemic outbreak of neonatal necrotizing enterocolitis (NEC). In addition, all cases of infants with this disease since 1964 were reviewed for comparison with the present outbreak. Efforts were made to determine a transmissible cause. No bacteria, virus, or fungus common to the infants involved in the outbreak was isolated. Neither could any other transmissible cause be implicated. It appears that the organisms involved in NEC are normal bowel flora that enter the peritoneum and blood stream through an altered gut mucosa.