Because of an increased incidence of necrotizing enterocolitis (NEC) temporally related to Klebsiella pneumoniae colonization noted in several infants, a study was undertaken to identify the source and to eradicate the infection. Twenty infants in the neonatal intensive care unit (NICU) and 51 in the well-baby nursery were prospectively studied. Cultures were done on all infants. In addition, cultures were done on all employees and parents who regularly visited. Our results showed an increased incidence of infection during this period--8.3% as compared with 4.79% overall. Colonization with K pneumoniae was eradicated, at least temporarily, by establishing a cohort system in both the NICU and the well-baby nursery and by the administration of antibiotics to all infants in these nurseries. Colonized parents and employees were also treated. Follow-up cultures showed clearing of Klebsiella, and the cohort system was abolished. Serotyping of all affected infants later showed that different strains were present, making it unlikely that there was a common source for the Klebsiella. (Initially the source of infection had been thought to be a mother whose premature triplets were colonized. We feel that the eradication of this organism and the subsequent decline in NEC was enhanced by the combination of cohorting and handwashing. The selected use of antibiotics may have prevented an extension of the outbreak, but this could not be proven.