Epidemiological fingerprinting of Klebsiella pneumoniae was performed by restriction endonuclease analysis (REA) of whole cell DNA. 11 isolates from 4 patients in an intensive care unit and 80 unrelated strains were examined in this study. DNA was cleaved with restriction endonuclease EcoR I, electrophoresed on 10% polyacrylamide gels, and restriction fragment patterns were visualized by silver staining. The analysis of small fragments within the cleavage patterns (SF-REA) yielded sufficient information for reliable strain identification. The gel patterns of unrelated strains exhibited marked differences by direct visual comparison. In contrast, the isolates from the ICU could only be subdivided into 2 types, supporting our suspicion of nosocomial infections in some of these patients. SF-REA was evaluated with regard to interstrain discriminatory ability, reproducibility, and practicability. Our results indicate that SF-REA may be used as a rapid, precise and reliable technique in typing K. pneumoniae strains.