An outbreak of colonization of 11 neonates with Enterobacter sakazakii occurred in a neonatal intensive care unit from the 10 September to 17 October 1984. During this period Ent. sakazakii was isolated from throat and rectal swabs and tracheal aspirates, but not from blood, of the neonates. The duration of colonization ranged from 2 to 8 weeks. The isolates were resistant to amikacin and to tobramycin, but sensitive to gentamicin. Four of the 11 colonized neonates had clinical signs of severe sepsis and one of meningitis and four died in spite of intensive chemotherapy. The source and the mode of spread of Ent. sakazakii remained unknown as it was not found on the hands of staff or in the inanimate environment of the unit. Ent. sakazakii may be implicated in severe infections in neonates and should be included when screening clinical specimens.