The role of blood groups as risk factors for breast cancer and their predictive value after radical surgery have been retrospectively evaluated in 315 breast cancer patients. The AB0 group distribution in these breast cancer patients was not significantly different from that of control women. Patients with blood group 0 showed a significantly lower risk of death than those with groups A, B and AB. Moreover, by using Cox's multivariate analysis, blood groups were shown to possess a predictive value independent of other known prognostic factors. These results suggest that also genetic factors could be involved in the pathophysiology of breast cancer growth and need to be further investigated.