The human ABO (H) blood group antigens are genetically determined carbohydrate structures found in different cells and tissues. The modulation in their expression in the course of human onto- and oncogenesis gives grounds for speculations concerning the role of ABO (H) antigens as molecules, participating in cell interactions, adhesion and inhibition events, as well as in tumor metastases. The deletion of antigenic determinants observed in a number of human carcinomas, and the epitope homology between the blood group antigens and the carcinoembryonic antigen, focuses on clarifying the diagnostic and prognostic value of the ABO (H) antigens as tumor-associated markers.