The prevalence of Helicobacter pylori infection varies between different geographic locations. The objectives of this study were to determine the prevalence of H. pylori infection in patients with gastrointestinal (GI) symptoms and to describe the association of H. pylori infection with demographic data, clinical diagnosis, and previous histories of patients. The study was carried out at the gastroenterology unit of Queen Sawang Wattana Memorial Hospital, Chon Buri, Thailand. The diagnosis of H. pylori infection was done by culture and rapid urease test on the gastric biopsy specimens of 112 patients. The overall prevalence of H. pylori infection by the gastric biopsy-based method was 58%. The prevalence of H. pylori infection in duodenal ulcer (DU) patients (75%) was significantly higher than in gastric ulcer (GU) patients (56.4%) and patients with gastritis (44.1%). A reverse correlation was observed between H. pylori infection and household income. The prevalence of H. pylori infection in patients who usually consumed unboiled water was 61.6%, which was significantly higher than in those who consumed boiled water (30.8%). We conclude that the prevalence of H. pylori infection in patients with GI symptoms is relatively high, and H. pylori infection is associated with DU disease. The data suggests that the household income and not boiling drinking water are related to the high H. pylori infection in our study.