SYNOPSIS Eight hundred and sixty-five patients with diabetes mellitus showed ABO blood group frequencies closely similar to those expected from the controls. Six hundred and sixteen patients with diabetes mellitus showed frequencies of secretion and non-secretion of the ABH(O) substances in the saliva closely similar to those expected from the controls. Four hundred and fifty-three patients with diabetes mellitus gave MN blood group frequencies very similar to those expected from the controls.There are certain diseases which show strong evidence of association with the ABO blood groups, notably duodenal ulcer with blood group 0 and carcinoma of the stomach with blood group A. However, examination of the literature produces conflicting results with regard to diabetes mellitus.Results of a combined series from Lancashire, Cheshire, and Oxford (McConnell, Pyke, and