Of 160 patients with pernicious anaemia, none had current duodenal ulceration, whereas in a random population of similar age and sex distribution some 5% would be expected to have a duodenal ulcer. Parietal-cell antibody was detected in serum from 8 of 169 men (4-7%) and from 2 to 31 women (6-4%) with duodenal ulceration. None of the 200 duodenal ulcer patients had antibody to intrinsic factor. The prevalence of these antibodies in duodenal ulcer patients was not significantly different from that in control subjects of similar age and sex distribution. The decreased prevalance of duodenal ulcer in pernicious anaemia patients implies that pernicious anaemia must be less prevalent in duodenal ulcer patients than in a random population; but it appears that this cannot be attributed to an absence of gastric autoimmunity in patients with duodenal ulcer. To resolve this disrepancy, we suggest that pernicious anaemia is determined not only by autoimmune reactions, but also by independent genetic and environmental factors which influence the state of the gastric mucosa.