PURPOSE: We have previously reported that the reverse transcriptase-polymerase chain reaction (RT-PCR) for cytokeratin 19 (CK 19) is a highly sensitive and specific method for detecting micrometas tases in the bone marrow of patients with primary breast cancer. Thus, we conducted this study to identify occult metastatic cells in the bone marrow of follow-up patients with breast cancer by using the RT-PCR for cytokeratin 19 after surgery. METHODS: We studied 42 patients with breast cancer who had no evidence of relapse, 3 patients who had a local relapse, and 7 patients who had distant metastases. Bone-marrow aspirates were taken from the single posterior iliac crest under local anesthesia anesthesia at a median of 45 months after surgery. RT-PCR for CK19 was done by using two 30-cycle rounds with nested primers. RESULTS: Occult metastases were found in 23% of the 52 breast-cancer patients. None of the bone-marrow aspirates showed any evidence of tumor involvement by using conventional cytology. RT- PCR was positive for CK 19 in 19% of the 42 patients without relapse, and in 40% of the 10 patients with local or distant relapse. CONCLUSION: RT-PCR for CK 19 is a sensitive method for detecting tumor cells in the bone marrow of follow-up patients with breast cancer after surgery. This assay may be useful in detecting metastastic disease, as well as in monitoring the effectiveness of systemic therapy. However, a large study with long-term follow-up is required in order to clarify its clinical significance and usefulness.