Background/Aims : Clarithromycin-resistance of Helicobacter pylori (H. pylori) is worldwidely increasing. However, culture which is one of means of determining the resistance has some problems such as prolonged time and low sensitivity. Clarithromycin-resistance of H. pylori is known to be caused by point mutation within 23S rRNA gene. We have evaluated its genetic mutations by polymerase chain reaction (PCR), and determined whether this technique is a useful tool in detecting clarithromycin-resistance of H. pylori. Methods: One hundred fourteen patients with H. pylori infection were treated with omeprazole-amoxicillin-clarithromycin (OAC) regimen. To detect substituted mutations (adenine-guanine) at sequence site 2142 or 2143 in 23S rRNA gene, PCR products were digested with Mbo II or Bsa I, respectively. Cultures and Epsilometer test (E-test) were also performed as a comparative test. Results: Mutation rate determined by PCR was 20.2% (23/114). A2142G mutation was observed in 20 of 114 patients, and three patients showed A2143G mutation. The treatment failed in 21 cases (18.4%), in which PCR detected the mutations in 23S rRNA gene. Clarithromycin-resistant rate identified by E-test was 28.8% (15/52) and culture positive rate of H. pylori was 45.6% (52/114). Conclusions : Clarithromycin-resistance was correlated with failure rate of treatment with OAC regimen. Clarithromycin-resistance could be detected easily by PCR and thus, PCR might be a useful test before antimicrobial treatment. (Kor J Gastroenterol 2000;36:450 - 456)