Heterotopic gastric mucosa in the duodenal bulb is a rare congenital disorder with varied clinical presentations. The mechanism of formation of a duodenal ulcer is failure of balance of the attack factor and the defense factor, which is the same as the mechanism of formation of a gastric ulcer. However, the true etiology of the duodenal ulcer remains unknown. Gastric mucosa can secrete gastric juice which injures itself, but the duodenal mucosa does not contain cells secreting a digestive enzyme. We assume that duodenal ulcers are caused by the presence of heterotopic gastric mucosa that can secrete gastric acid. This study was designed to assess the prevalence and associations of heterotopic gastric mucosa in duodenal ulcers. The present study included 137 patients who underwent biopsy or resection of duodenal ulcer. We detected gastric foveolar metaplasia due to inflammation from a heterotopic gastric mucosa using immunohistochemical staining. Heterotopic gastric mucosa consists of foveolar epithelium (MUC5AC-positive) and fundic gland (H⁺K⁺ ATPase-positive parietal cells, pepsinogen I-positive chief cells and MUC6-positive mucous neck cells), whereas gastric metaplasia is composed of foveolar epithelium without fundic glands. These specimens were stained with toluidine blue for detection of Helicobacter pylori infection. Among the 137 patients with duodenal ulcer, 76 cases (55%) had heterotopic gastric mucosa in the obtained specimens, and Helicobacter pylori was found in 45 cases (59%,45/76) among those with heterotopic gastric mucosa. Our results suggest that heterotopic gastric mucosa was strongly associated with concurrent duodenal ulcer.