BACKGROUND: For the treatment of peptic ulcer diseases infected with Helicobacter pylori (H. pylori), amoxicillin, clarithromycin, and metronidazole are most commonly used. Recently the resistant rates against these antibiotics have been increased and this has become one of the major causes of treatment failure. It was recently demonstrated that metronidazole resistance is associated with mutation in rdxA gene that encodes an oxygen-insensitive NADPH nitroreductase. The aim of this study was to investigate the mutations of the rdxA gene for the metronidazoleresistant H. pylori isolates from Guri City, Korea. METHODS: H. pylori strains were isolated from gastric biopsy specimens from patients diagnosed as having peptic ulcer or gastric carcinoma in Hanyang University Guri Hospital. Antimicrobial susceptibility testing was performed by the modified broth microdilution method. Resistance was defined as metronidazole minimum inhibitory concentration (MIC) being more than 16 ug/mL Three metronidazole-sensitive and 10 metronidazole-resistant strains were selected to detect mutations in the rdxA genes by direct sequencing of PCR products. RESULTS: Of the 266 clinical isolates of H. pylori that were isolated from 1996 through 2001, 90 isolates (33.8%) showed metronidazole resistance. The frequency of nucleotide substitutions of the rdxA gene of 10 metronidazole-resistant strains (25-33/strain) was not so different from that of the three metronidazole-sensitive strains (22-26/strain). Stop signals generated by nucleotide substitution, insertion, or deletion, were found in 5 metronidazole-resistant strains (50%), but not in 3 metronidazole- susceptible strains. CONCLUSION: The present study confirms that the presence of mutations on the rdxA gene causing a premature stopping of the inferred RdxA protein is associated with metronidazole resistance. But other genes or mechanisms might be implicated in the generation of metronidazole resistance and further investigations are needed.