Two unselected series of 528 Chinese and 539 Scottish patients with duodenal ulcer were compared. In both races, early-onset patients (symptoms began before age 30 years) in contrast to late-onset patients (symptoms began after age 30 years) constituted significantly more males, more patients with positive familial dyspepsia, more acid hypersecretors, and more gastrointestinal bleeding, but, unlike late-onset patients, there was no significant blood group O predominance, and their females did not have an older onset age than males. Among early-onset patients, Scottish patients developed symptoms at a significantly later age and had significantly more acid hypersecretors than Chinese, whereas among late-onset patients the Scots had significantly more patients with positive familial dyspepsia, in whom the frequency of acid hypersecretion was increased. These findings indicate that (i) early and late-onset duodenal ulcers are distinct subgroups, with many features common to both Chinese and Scots despite different ethnic backgrounds, suggesting the occurrence of similar pathophysiological mechanisms in the two races, (ii) for early-onset disease, these mechanisms appear to operate sooner in the Chinese, (iii) in the Scottish patients, familial late-onset duodenal ulcers with acid hypersecretion form a substantial and distinct subgroup.