Background/aims: Proton pump inhibitors are mainly metabolized by cytochrome P450 2C19 in the liver. Recently, some studies have shown that the acid suppressing effect of proton pump inhibitors are influenced by a functional polymorphism of cytochrome P450 2C19. The aim of the present study was to investigate the effect of cytochrome P450 2C19 polymorphism on Helicobacter pylori eradication in patients who received proton pump inhibitors based triple therapy. Methods: We determined the incidence of cytochrome P450 2C19 genotypes and the effect of cytochrome P450 2C19 genotypes on Helicobacter pylori eradication rates in 105 patients with Helicobacter pylori-positive chronic gastritis. Upper endoscopic procedure and gastric biopsies were performed in all patients. Helicobacter pylori was demonstrated histologically. Lansoprazole, amoxicillin and clarithromycin twice a day for 14 days were prescribed for those found to be infected with Helicobacter pylori. More than one month after the medication, a C-13 urea breath test was conducted to examine the success or failure of the eradication treatment. Cytochrome P450 2C19 polymorphism was analyzed by the polymerase chain reaction restriction fragment length polymorphism method. Results: The genotypes of cytochrome P450 2C19 were classified into the three groups, as rapid extensive metabolizer, intermediate metabolizer and poor metabolizer. In our patient population, the frequencies of rapid extensive metabolizer, intermediate metabolizer and poor metabolizer were 72%, 23% and 5%, respectively. The eradication rate was 70.0% for rapid extensive metabolizer, 92% for intermediate metabolizer and 80% for poor metabolizer. The eradication rate was highest in intermediate metabolizer patients. Conclusions: The present study confirmed the low eradication rate for rapid extensive metabolizer. Our findings provide evidence that the cytochrome P450 2C19 genotype is useful to predict the success of treatment. For the rapid extensive metabolizer group, alternative regimens can be tried to increase the Helicobacter pylori eradication rates.