The present review focuses on the methodology and clinical significance of new diagnostic approaches to identify micrometastatic breast cancer cells present in bone marrow (BM), as a frequent site of overt metastases. Using monoclonal antibodies (mAbs) to epithelial cytokeratins (CK) or tumor-associated cell membrane glycoproteins, individual carcinoma cells can be detected on cytologic BM preparations at frequencies of 10(-5) to 10(-6). Prospective clinical studies have shown that the presence of these immunostained cells is prognostically relevant with regard to relapse-free and overall survival. The current interest in autologous bone marrow transplantation in patients with solid tumors further underlines the need for screening methods that allow the detection of minute numbers of residual tumor cells in the transplant. Although the development of new molecular detection methods based on the amplification of a marker mRNA species by the polymerase chain reaction technique is a very exciting area of research, the clinical significance of this approach needs to be demonstrated in prospective studies. The immunocytochemical assays may be, therefore, used to improve tumor staging with potential consequences for adjuvant therapy. Another promising clinical application is monitoring the response of micrometastatic cells to adjuvant therapies, which, at present, can only be assessed retrospectively after an extended period of clinical follow-up. The extremely low frequency of BM tumor cells greatly hampers approaches to obtain more specific information on their biological properties. The available data indicate that these cells represent a selected population of cancer cells which, however, still express a considerable degree of heterogeneity with regard to the expression of MHC class I antigens, adhesion molecules (EpCAM), growth factor receptors (EGF receptor, erb-B2, transferrin receptor), or proliferation-associated markers (Ki-67, p120). Regardless of the detection technique applied, there is an urgent demand for large multicentre trials, in which standardized methods are related to specified clinical outcomes.