Immunocytochemistry using tumour-associated monoclonal antibodies has led to improvements in the ability to detect occult breast cancer cells in bone marrow aspirates and peripheral blood. Nevertheless, the immunocytochemistry method needs to be further developed before it can be used routinely in the clinic. Reverse transcription polymerase chain reaction assays (RT-PCR) that screen for carcinoma-specific expression of mRNA in bone marrow and blood have been developed. However, it is not yet clear whether the most frequently employed RT-PCR assay for cytokeratin 19 has the specificity required to be safely used in the clinic. In spite of many unsolved standardization problems with micrometastatic detection methods, recent data show that the presence of occult tumour cells in the bone marrow at diagnosis and in the reinfused autograft after high-dose therapy appears to increase the rate of recurrence in the patients.