An epidemiological investigation of an outbreak of Acinetobacter baumannii among patients on 2 closely related intensive care units (ICU) was performed by molecular typing with interrepeat polymerase chain reaction (interrepeat PCR). 31 A. baumannii isolates obtained from 15 ICU patients were characterized. All patients were infected or colonized with A. baumannii. After identification of the outbreak, 6 environmental isolates were collected from tap-water, sinks and cleaning detergents. PCR fingerprinting identified 3 genotypes among the outbreak-related strains. One predominant genotype was demonstrated in 14/15 patients and this genotype was also found among all environmental isolates. The cluster of A. baumannii represented an outbreak of 1 genotype, suggesting cross-contamination. The finding of the identical genotype among all environmental strains indicated a common environmental source causing the outbreak. The outbreak was controlled after reimplementation of an effective disinfection of workplace surfaces. This survey proved interrepeat PCR to be a rapid and reliable method to differentiate A. baumannii strains, thereby allowing epidemiological surveillance of large amounts of strains and early interventions to control outbreaks.