OBJECTIVE In colorectal cancer (CRC) patients, tumour recurrence is common following potentially curative surgery for liver metastases. This may be caused by occult tumour cells present at the time of surgery. Dissemination of micrometastatic cells may occur early in patients with solid cancer, and micrometastases may signify a poor prognosis. The aim of the present study was to evaluate the frequency of micrometastatic cells in the bone marrow of patients with potentially resectable liver metastases. METHODS Twenty millilitres of bone marrow was aspirated from both anterior iliac crests from 48 patients. Mononuclear cells were isolated and incubated with superparamagnetic Dynabeads coated with an anti-epithelial monoclonal antibody (MOC31). Magnetically selected cells were identified by light microscopy as cells with bead rosettes (>5 beads/cell). RESULTS Micrometastatic tumour cells were identified in four of 48 (8%) patients who all had their liver metastases surgically removed. Two of the four died after 17 and 18 months, respectively, whereas two are alive after 10 and 12 months. None of the 19 inoperable patients had micrometastases. CONCLUSIONS The frequency of bone marrow micrometastases in patients with clinically isolated liver metastases from CRC was low. This is biologically interesting, but bone marrow status should not affect current treatment protocols.