BACKGROUND: The pathogenesis of non-ampullary duodenal epithelial tumors (NADETs) is not fully understood. NADETs that express gastric-type mucin phenotypes (G-NADETs) are noteworthy because of their high malignancy. Gastric foveolar metaplasia, from which G-NADETs originate, protects the duodenal mucosa from gastric acidity. As gastric acid secretion is affected by endoscopic gastric mucosal atrophy (EGMA), we hypothesized that EGMA would be associated with G-NADETs. AIM: To evaluate the association between EGMA and the occurrence of G-NADETs. METHODS: This cross-sectional retrospective study investigated the relationship between EGMA and NADETs in 134 patients. The duodenum was divided into parts 1 (bulb), 2 (superior duodenal angle to the papilla), and 3 (anal side of the papilla to the horizontal part). The effects of gastric acidity and presence of Brunner's glands were considered. EGMA was divided into types C (no or mild atrophy) and O (severe atrophy). Mucin phenotype expressions in NADETs were divided into gastric, intestinal, gastrointestinal, and unclassifiable. RESULTS: When NADETs were classified according to EGMA, 105 were classified as type C and 29 as type O. G-NADETs were present in 11.9% (16 cases) of all cases, and all 16 cases were of type C. Among G-NADETs, 93.8% (15 cases) were present in part 1 or 2. There was an association between G-NADETs and type C in part 1, and 50.0% (eight of 16 cases) of G-NADETs were associated with a current or previous CONCLUSION: G-NADETs were significantly associated with type C. Gastric acidity and Brunner's gland growth may be associated with G-NADETs.