• The incidence of the four blood types O, A, B, and AB was studied in 1,301 patients with duodenal ulcer, 469 with gastric ulcer, and 69 patients with both gastric and duodenal ulcers. The three distributions so obtained were compared with the distribution found in a reference of 8,767 people without ulcers. Type O was more frequent among the ulcer patients than in the reference group, and calculations of probability showed that the association between type O and peptic ulcer was statistically significant. It was shown not to be secondary to any influences of sex, age, rhesus blood type, or occupation. Among a group of 529 patients who had histamine tests, however, the frequency of type O was significantly greater among those who reacted with an increase of free hydrochloric acid than among those in whom no increase of acid occurred.
Buckwalter JA, Wohlwend EB, Colter DC, Tidrick RT, Knowler LA. PEPTIC ULCERATION AND ABO BLOOD GROUPS. JAMA. 1956;162(13):1215-1220. doi:10.1001/jama.1956.02970300015006