Background: The CYP2C19 genotype has been found to be an important factor for peptic ulcer healing andH. pylori eradication, influencing the efficacy of proton pump inhibitors (PPIs) and the pathogenesis of gastriccancer. The aim of this study was to investigate clinical correlations of the CYP2C19 genotype in patients withgastritis, peptic ulcer disease (PUD), peptic ulcer bleeding (PUB) and gastric cancer in Thailand. Materialsand Methods: Clinical information, endoscopic findings and H. pylori infection status of patients were assessedbetween May 2012 and November 2014 in Thammasat University Hospital, Thailand. Upper GI endoscopy wasperformed for all patients. Five milliliters of blood were collected for H. pylori serological diagnosis and CYP2C19study. CYP2C19 genotypes were determined by polymerase chain reaction (PCR) and restriction fragment lengthpolymorphism analysis (RFLP) and classified as rapid metabolizer (RM), intermediate metabolizer (IM) or poormetabolizer (PM). Results: A total of 202 patients were enrolled including 114 with gastritis, 36 with PUD, 50 withPUB and 2 with gastric cancer. Prevalence of CYP2C19 genotype was 82/202 (40.6%) in RM, 99/202 (49%) inIM and 21/202 (10.4%) in PM. Overall H. pylori infection was 138/202 patients (68.3%). H. pylori infection wasdemonstrated in 72% in RM genotype, 69.7% in IM genotype and 47.6% in PM genotype. Both gastric cancerpatients had the IM genotype. In PUB patients, the prevalence of genotype RM (56%) was highest followed byIM (32%) and PM(12%). Furthermore, the prevalence of genotype RM in PUB was significantly greater thangastritis patients (56% vs 36%: p=0.016; OR=2.3, 95%CI=1.1-4.7). Conclusions: CYP2C19 genotype IM was themost common genotype whereas genotype RM was the most common in PUB patients. All gastric cancer patientshad genotype IM. The CYP2C19 genotype RM might be play role in development of PUD and PUB. Furtherstudy in different population is necessary to verify clinical usefulness of CYP2C19 genotyping in developmentof these upper GI diseases.