The possibility of an association of certain disorders with the Rhesus and MN blood groups was investigated in 1,030 duodenal ulcer, 348 gastric ulcer, 170 gastric carcinoma, 540 rheumatic fever, and 1,153 diabetes mellitus patients. This was done by comparing the blood-group frequencies observed in these patients with those observed in 2,186 controls; the differences were examined for statistical significance by appropriate statistical methods. It was found that the liability to duodenal and gastric ulceration was statistically increased in persons of the presumptive Rhesus genotype R1R2. There was evidence, of less statistical significance, of an association of the Rhesus blood groups with gastric carcinoma and rheumatic fever. A statistically significant increased liability of persons of NN genotype to rheumatic fever was found.