Secretory and clinical characteristics of 339 hospital patients with uncomplicated duodenal ulcers, 182 with pyloro-duodenal stenosis, 124 operated upon in Copenhagen by proximal vagotomy and 174 in whom bleeding or perforation was the reason for admission were studied. The results indicate that all duodenal ulcer subjects secrete more acid than normal, but that in approximately half of them acid secretion to maximal stimulation does not rise outwith the range of normal and most of these are of O blood group. These patients are classified as Type I duodenal ulcer. The character of acid hypersecretion beyond two standard deviations of the mean of the maximal acid output in normal subjects appears to be determined mainly by the possession of A, B and AB blood group: these hypersecreting ulcer patients are classified as Type II. The complications of perforation and bleeding and stenosis are linked mostly to the O group subjects and not to acid hypersecretion as such, but the strongest family prevalence is in the A, B, AB series and in them also the disease appears earlier. A hypothesis is presented to explain the aetiology of the two types.