ABSTRACTBackground.Metronidazole is one of the most commonly used antimicrobial agents for the treatment ofHelicobacter pyloriinfection. Resistance to metronidazole has been reported worldwide but with a wide range of prevalence. We started using the classical triple therapy (bismuth, tetracycline, and metronidazole) forH. pyloriinfection in 1991 but recently have experienced a decline in its efficacy in curing the infection. Thus our aim was to investigate in a single center the prevalence of metronidazole‐resistantH. pyloriover a period of 5 years.Materials and Methods.A total of 1,015 differentH. pyloristrains collected over a period of 5 years were tested for sensitivity against metronidazole, ampicillin, tetracycline, and imipenem. Antibiotic sensitivity was tested by the disk diffusion and agar dilution methods. To elucidate further the possible relationship between these metronidazole‐resistant strains, genomic DNA digestion by theHaeIII endonuclease and ribotyping were undertaken in a selected group of isolates.Results.In 1991, 29 of 132 (22.0%) tested strains ofH. pyloriwere found to be resistant to metronidazole. Since our initiation at that time of a triple therapy of bismuth, metronidazole, and tetracycline, the prevalence of metronidazole‐resistant strains rose rapidly to 73.2% in 1995. AllH. pyloriisolates were sensitive to ampicillin, tetracycline, and imipenem. A high degree of genomic heterogeneity was found among these isolates. Thus it is unlikely that the resistant strains ofH. pyloriwere originated from a single clone.Conclusions.This study shows a rapid increase in metronidazole‐resistantH. pyloriwith the use of an anti‐Helicobacterregimen that contains metronidazole. We anticipate that the efficacy of metronidazole‐containing anti‐Helicobacterregimens will decline with the rapid rise in resistant strain