Abstract A study of 864 patients with occlusive disease and 246 with ectatic disease was made with reference to their ABO blood groups. The results confirmed the findings of other workers that occlusive disease and myocardial infarction occur more commonly in group-A individuals and that blood group O seems to confer some immunity to occlusive disease. There was also a probable correlation between group A and ectatic disease, but belonging to group O did not seem to confer any protection in this disease. It was also found that the incidence of myocardial infarction was much lower in ectatic disease than in occlusive disease. It is suggested that ectatic disease and occlusive disease are two distinct entities.