551 clinical records of breast cancer patients were retrospectively analysed. Several parameters have been examined: age at presentation, distribution of stages at the time of diagnosis, incidence of any progression and "progression- free" interval after primary treatment, incidence of distant dissemination and distant meta- free interval, cancer mortality, time of survival from presentation and time of survival from detection of distant metastases (stage at presentation being taken into account in all evaluations). The results within various ABO blood groups were mutually compared. There were no substantial differences in these parameters within different blood groups. Immunological point of view, dealing with possible loss or modifications of ABH (O) isoantigens on tumour cells and the immune response to these alien antigens, is discussed.