The incidence of breast cancer continuously increases in developed countries. The introduction of screening methods such as mammography or ultrasound lead to higher proportion of early diagnosed tumors. However, even in early stage tumors occult neoplastic cells can spread to the organism. Such tumor cells are very likely precursors of distant metastases. Using several monoclonal antibodies against epithelial mucins or cytokeratins on the cell surface could be detected one tumor cell among 10 5 or 10 6 of normal bone marrow cells. These cells are not detectable by routine histopathologic exam. More sensitive but also more costly and technically demanding are PCR assays. The sensitivity might reach almost 1:10 7. Prospective clinical trials using immunocytochemistry have shown that the presence of stained cells in bone marrow is clearly associated with shorter disease free survival and overall survival. In the near future we may use the bone marrow examination for the presence of occult tumor cells in order to improve current staging system or as a surrogate marker in the decision-making in regard to adjuvant systemic therapy or in the assessment of efficacy of adjuvant treatment. The review summarizes contemporary knowledge assembled in preclinical and clinical studies.

        Key words: breast cancer, bone marrow, cytokeratin, mammaglobin, minimal residual disease.