Mainstream physicians rarely look at this but there has been a movement in the naturopathic medical community to look at the research on this topic and apply it to their patients.
Most of the research is being done by medical anthropologists and medical doctorates in institutions, and the publications are printed in the more obscure journals such as "The European Journal of Dentistry", and the "International Journal of Epidemiology".
Clinicians typically do not read such specific journals, these are most frequently read by researchers in their specific fields.
Among medical anthropologists, it has been noted that people with blood type A have higher rates of esophageal, pancreatic and stomach cancers, while those with blood type O have higher rates of peptic and duodenal ulcers.
In a recent study, researchers looked several groups of women and found a small preponderance of blood type A in the breast cancer groups.
There has been very good research showing that patients with blood type O who have breast cancer do better than other blood type patients, significantly better. The blood types that fair the worst outcomes tend to be AB.
The list in longterm survival rates from best to worst is O>A>B>AB. Note, this is statistics and individual cases always brake the mold. So even though women with blood type A have a slightly greater chance or getting breast cancer, they tend to fair better than those with B or AB, but not as well as those with O.
For more information, read Dr. D'adamo's Eat Right for Your Blood Type. While he does cite research, his ideas are not recognized by mainstream medicine. However, I have found his advice useful clinically and his theories make sense according to the research I have cited in this article.