The biological and clinical significance of circulating tumor cells (CTC) in the peripheral blood of breast cancer patients is not known. To study this question, we used a direct visualization assay to correlate the number of CTC with disease stage and progression. The CTC were enriched from the nucleated cell fraction by filtration and enumerated visually following immunostaining with anti-cytokeratin 8 (CK8) antibody CAM 5.2. In mixing experiments, we achieved a limit of detection of 5 MCF7 cells per 5 ml of blood or 5 x 10(7) peripheral blood leukocytes (PBL). We did not detect CTC in any control subjects (0/20). In 131 breast cancer patients, we found a higher incidence of CTC in patients with distant metastatic 36/51 (71%) than those with node-positive 17/36 (47%) (p = 0.026), or node-negative 17/44 (39%) (p = 0.001) disease. The distribution of the highest numbers of CTC observed in individual patients by repeated sampling over time ranged from 1 to 700 per 5 ml of blood with a trend toward higher numbers in those with distant metastases. In comparison with previous studies of equal specificity, based on a similar absence of CTC in controls, we report a higher incidence of CTC in node-negative and node-positive patients, suggesting a more frequent detection of CTC by our approach. This higher incidence was achieved, in part, by repeated sampling of our study population over time. Our results support the concept that CTC can be detected and enumerated in peripheral blood and that this minimally invasive assay merits further evaluation as a potential prognostic indicator and marker of disease progression.