A case-control study of 80 patients with duodenal ulcer and 80 community controls was undertaken to compare these two groups regarding social and environmental factors including socio-economic status, marital status, country of birth, childhood and family factors. An increased risk of duodenal ulcer was associated with status incongruity in males; the male patients, when matched on suburb of residence, having lower status occupations than controls, and when matched on occupational status, tending to have lower educational levels than controls. Being unmarried was associated with an increased risk of duodenal ulcer in females. The number of siblings was also a risk factor in the male patients, those with more than 5 siblings being at an increased risk. Factors not associated with duodenal ulcer included country of birth, childhood happiness, sibling sequence and family stability. In conclusion, the factors associated with chronic duodenal ulcer were found predominantly in adult life and could produce their adverse effects by causing chronic stress which may be more relevant than acute stress as produced by major life events.