Gastric epithelium in the duodenum: its association with Helicobacter pylori and inflammation. Duodenal biopsy specimens from 471 adults and 47 children were examined to determine the prevalence and distribution of gastric epithelium in the duodenal bulb in relation to age, gender, gastroduodenal inflammation, smoking, alcohol and consumption of nonsteroidal anti-inflammatory drugs (NSAID). Gastric metaplasia was present in the anterior wall duodenal biopsy specimen in 31%, was significantly less common in patients under 17 than in adults, and was more common in males than females. In sixty two adults who underwent multiple radial duodenal biopsy gastric metaplasia was randomly distributed around the duodenal circumference; sixty three per cent of the patients with gastric metaplasia found on multiple biopsy were detected by just the anterior biopsy. Gastric metaplasia was not obviously associated with alcohol, cigarette, or NSAID consumption. While the presence of gastric metaplasia was associated with adulthood, male sex, and low fasting gastric juice pH, its extent was associated with active duodenitis and Helicobacter-associated gastritis. On logistic regression, gastric metaplasia in the duodenum and gastric Helicobacter pylori were independent predictors of active duodenitis, but were not significantly associated with inactive duodenal inflammation. H pylori was observed in duodenal biopsy specimens from 32 patients, all with active duodenitis; bacteria were present only on foci of gastric metaplasia, and were more likely to be seen when the metaplasia was extensive. It is proposed that inflammatory injury to the duodenal mucosa by H pylori may stimulate the development of further gastric metaplasia, and that the area of duodenum susceptible to colonisation with H pylori may therefore increase progressively until mucosal integrity is compromised and ulceration supervenes. 