The clinical records of 551 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 metastasis-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 compared. There were no substantial differences in these parameters within different blood groups. The possible modifications of ABH (O) isoantigens on tumour cells and the immune response to these alien antigens is discussed.