Compromise of the gastric acid barrier may facilitate bacterial invasion of the lower intestinal tract and promote the development of colonic neoplasia. Our study tested the associations between histopathologic abnormalities of the upper and lower gastrointestinal tract in patients undergoing bidirectional endoscopy.The Inform Diagnostics database is a national electronic repository of histopathologic records of patients distributed throughout the USA. A case-control study of 302,061 patients, of whom 163,168 had colon polyps, evaluated whether the occurrence of colonic polyps was influenced by the presence of the following gastroesophageal diagnoses: gastric H. pylori infection, gastric intestinal metaplasia, fundic gland polyps, and gastric hyperplastic polyps. The influence of individual diagnoses on the occurrence of colonic polyps was expressed as odds ratios with their 95% confidence intervals.The odds ratio for tubular adenomas being associated with gastric H. pylori was 1.53 (1.49-1.58), with intestinal metaplasia 1.65 (1.59-1.71), with fundic gland polyps 1.49 (1.45-1.54), and with gastric hyperplastic polyps 1.85 (1.75-1.96). The odds ratio for sessile serrated polyps being associated with gastric H. pylori was 1.03 (0.96-1.10), with intestinal metaplasia 1.21 (1.13-1.30), with fundic gland polyps 1.79 (1.69-1.89), and with gastric hyperplastic polyps 1.52 (1.35-1.71.A diminished gastric acid barrier function, which occurs in various upper gastrointestinal diseases associated with lowered gastric acid output, may promote the development of colonic neoplasia.