Intragastric bacterial colonization is well known in pernicious anaemia (PA), but its consequences have rarely been investigated. We have studied the clinical history, blood samples, and endoscopic biopsies from the stomach and duodenum of 80 patients with PA. In a random subgroup of 22 patients gastric juice was collected for aerobic culture and for estimation of nitrate, nitrate-reducing bacteria, nitrite, and N-nitrosamines; duodenal juice was studied in parallel in eight of these subjects. Gastric and duodenal juice had high bacterial counts; faecal organisms were found in 14 patients. The mean count of nitrate-reducing bacteria was significantly higher than in a control group of patients with peptic ulcer disease (p less than 0.001), as was the nitrite concentration (p less than 0.001). Thirty-three of the 80 patients had gastric dysplasias; 1 early gastric carcinoma was also found. Duodenitis was present in 39 out of 80 cases, in 6 associated with partial villous atrophy. A history of malabsorption and/or chronic intermittent diarrhoea was obtained significantly more often from patients with duodenitis. Four patients developed acute gastroenteritis shortly before or during the time of the study, two having a salmonella infection. Bacterial overgrowth in PA may be facilitated by altered immunological conditions, since low serum levels of IgA and IgG were found in this patient group.