Objective: To determine whether a diet with a high calcium content slows the healing of duodenal ulcers. Whether this negative effect is due to calcium, which stimulates acid secretion in humans, is not known. Methods: Case-control study comparing calcium intake in 78 patients with recently diagnosed duodenal ulcers, with the intake of 151 community controls matched for sex and age. Logistic regression analysis was used to calculate relative risks for successive quintiles of calcium intake. Results: An increase in relative risks was noted for ascending quintiles of calcium intake. After controlling for energy intake, obesity, smoking and social class, the relative risk of duodenal ulcer in those in the highest quintile of calcium intake compared with those in the lowest was 4.3 (confidence limits 1.3-14.6). Conclusions: The strength and temporal precedence of this association, the evidence of a dose-response effect and a possible biological mechanism suggest that high calcium intake may play a role in duodenal ulcer development.