In 95% of patients with primary breast cancer, the extent of metastases cannot be proven by conventional methods. Nevertheless, more than 50% of these patients have a relapse within five years. To improve the predictive value for recurrency, we examined bone marrow aspirates of 128 patients with primary breast cancer. Bone marrow aspirates from 2-6 sites of the skeleton (iliac crest and sternum) were taken as well as biopsies for histological examination. The immunohistochemical studies were carried out on interphase smears and stained with cytoceratin antibodies (PKK 1) and antibodies against tumor-specific antigen TAG 12 (12 H 12). All patients were screened for distant metastases (X-ray, ultrasound, bone scan). Tumor cells and micrometastases in bone marrow were detected in 41 patients (32%). Their presence was correlated to other prognostic factors (tumor size, lymph node status, oestrogen/progesterone receptors). The median duration of follow-up was 39.5 months. 14 patients (45%) in the tumor cell positive group relapsed, compared to only 4 out of 36 patients in the tumor cell negative group. In 29% we found bone metastases. The relapse free interval was shorter for patients with micrometastases (8 vs. 15.8 months). The presence of tumor cells in bone marrow aspirates detected at the time of primary surgery, is a useful prognostic factor and a good predictor of metastases and may help in selecting patients for systemic adjuvant treatment.