The aim of this study was to define the frequency and clinical relevance of cytokeratin positive metastatic tumor cells in the peripheral circulation of patients with stage IV breast cancer. Peripheral blood was collected from 32 consecutive patients with metastatic breast cancer and 23 healthy donors. Tumor cells were enriched using positive selection with anti-HEA125-microbeads and cytospins were prepared of the positive selection eluate. Slides were incubated with a Fab2 fragment of the pancytokeratin antibody A45-B/B3 conjugated with alkaline phosphatase (AKP) and a CAM5.2-AKP monoclonal antibody and developed with an alkaline phosphatase anti-alkaline phosphate reaction (APAAP). All samples were evaluated using light microscopy and an automated image analysis system. In 8/32 (25%) patients cytokeratin positive (CK+) cells could be detected after anti-HEA125 enrichment in the peripheral blood whereas in none out of 23 healthy donors. One to 1000 (median 5) positive cells per patient sample were observed and cluster of tumor cells in one patient. Automated image analysis was as powerful in detecting micrometastases as conventional light microscopy. All patients with CK+ cells in the peripheral circulation (8/8, 100%) showed progressive disease at the time-point of blood draw whilst only 9/24 (37.5%) showed disease progression without detection of positive cells. The median overall survival of CK+ patients was 4+/-2 months compared to 13+/-7 months of CK- patients (p<0.001). CK+ cells are detectable in the peripheral circulation of 25% of patients with metastatic breast cancer after positive selection with anti-HEA125. Detection of tumor cells in the peripheral circulation might be correlated with progression of disease and shorter overall survival.