The incidence of breast cancer is increasing dramatically in the developed countries. Although attention is being paid to diagnose breast cancer early through screening programs, still a significant number of patients classified to have a localised disease at diagnosis, later experience a systemic recurrence. Therefore, more sensitive and reliable methods to detect the early spread of the disease, permitting the early use of additional systemic therapy, seem justified. New treatment strategies, such as immunotherapy employing monoclonal antibodies against breast cancer cells, is promising. Since such a treatment is most efficient on patients with limited disease, sensitive methods to monitor the therapeutic effect are needed. Immunocytochemistry using tumour associated monoclonal antibodies and reverse transcription polymerase chain reaction assays (RT-PCR), that screen for carcinoma-specific expression of mRNA in bone marrow and blood, have been developed. Many standardisation problems with the detection methods currently in use are unsolved. Despite this, we discuss here recent data showing that the presence of occult tumour cells in the bone marrow is a prognostic factor in patients with breast cancer.