Thirty-seven isolates of extended-spectrum beta-lactamase-producing (ESBL) Klebsiella pneumoniae implicated in five nosocomial outbreaks (I-V) on three distinct wards of our hospital were compared using capsular typing, biotyping, antibiotyping, enzyme electrophoresis typing and DNA macrorestriction analysis with Xba I resolved by pulsed-field gel electrophoresis. The isolates from each outbreak had common phenotypic and genotypic characteristics indicating that they were related epidemiologically. Isolates from outbreaks I (four patients) and V (13 patients), although they occurred in two different wards (neurology and surgery) and three years apart, produced the same ESBL with a pI of 7.8 (SHV-4) and were of serotype K25. The Xba I patterns were closely related. The isolates of outbreaks II (seven patients), III (four patients) and IV (seven patients), which occurred in a single surgical intensive care unit, produced an ESBL with a pI of 6.3 (TEM-3). Isolates from outbreaks III and IV, which occurred six months apart, were of serotype K68 and had similar Xba I patterns suggesting that the two outbreaks were due to a single strain which persisted endemically in the ward. The isolates from outbreak II were of serotype K62, and had distinct characteristics from the two later outbreaks. The Xba I patterns of the isolates from outbreaks "I and V', II and "III and IV' had Dice similarity coefficients under 40% showing that the three groups were genetically distant. DNA macrorestriction analysis was a useful complement to phenotypic methods for identifying K. pneumoniae strains responsible for outbreaks harbouring a common ESBL.