A set of 103 epidemiologically well-defined Acinetobacter baumannii isolates obtained from nine hospital outbreaks and 21 unrelated strains were characterized by pulsed-field gel electrophoresis (PFGE) of total genomic DNA digested with ApaI. Among outbreak strains, eight different patterns and five possible variants were identified by PFGE. Results were compared with those from traditional typing methods such as plasmid profile analysis, antimicrobial susceptibility, and biotyping. Plasmid analysis revealed six different and two related patterns; one outbreak strain lacked plasmids. A total of 16 of the 21 unrelated strains harbored plasmids and exhibited unique patterns. Epidemiologically unrelated strains were placed into only two biotypes and had similar antimicrobial susceptibility patterns but were clearly distinguished by PFGE. PFGE of A. baumannii chromosomal DNA yielded reproducible and easily readable results and showed excellent discriminatory power. However, plasmid profile analysis may provide a cost-effective first step in epidemiological typing of A. baumannii isolates obtained from well-defined hospital outbreaks.