An outbreak of multiresistant Klebsiella pneumoniae occurring in a neonatal intensive care unit is described. All infections developed at least 5 days after admission to the unit (range, 5-40 days). Four infants had septicaemia and one had urinary tract infection. Three of the infected infants died. All klebsiella isolates were resistant to ampicillin, cefotaxime, cefuroxime, co-amoxiclav, mezlocillin, chloramphenicol, gentamicin, and ceftazidime (except in two); all were susceptible to imipenem, amikacin and quinolones. An extensive case-control study identified the following significant risk factors for colonization: prematurity; presence of indwelling catheters; previous antibiotic treatment; and parenteral nutrition. The outbreak was controlled with re-emphasis on strict handwashing practices, cohorting, closure of the unit to outborn admissions, and changing the regimen of empirical antibiotic therapy. Physicians should be aware of multiresistant Klebsiella spp. and change treatment whenever clinically indicated, even before culture results are available.