The incidence of necrotizing enterocolitis (NEC) in our neonatal unit has varied from 4.7% to zero to 4.4% during three time periods. Simultaneously, significant changes have occurred in the spectrum of bacterial species in the gastrointestinal tract of unaffected infants in the same unit. During the first period of increased attack rate, 82% of gastric and 88% of fecal Enterobacteriaceae were E. coli and K. pneumoniae. When the attack rate decreased the frequencies were 11% (gastric) and 47% (fecal), and P. mirabilis was retrieved with increased frequency. The return of E. coli and K. pneumoniae as the dominant organisms was associated with an increase in NEC. Infants with NEC, compared with controls, had a statistically significant increased frequency of retrieval of E. coli and K. pneumoniae from gastric and fecal samplings. The data suggest an active role for certain enteric bacteria in the pathogenesis of NEC.