The goal of this pilot study was to determine in patients with operable breast cancer the incidence of breast cancer cells present in the blood, the clearance rate after surgical resection of the primary tumor, and the incidence of patients with persistent cancer cells in the blood after the primary tumor was removed. Twenty-one patients with operable breast cancer had 15 ml venous blood obtained twice prior to surgery and after surgery at 2, 4, 8, 12, 24, and 48 hours and also on days 7 and 14. Immunomagnetic selection of malignant cells was performed on each sample. Cells were then fixed on slides and immunocytochemistry performed on the collected cells. Cells that had a rosette of magnetic beads, cytoplasmic staining for keratin, and malignant morphology were counted as breast cancer cells. Eighteen of 19 of patients had cancer cells detected in at least one of the two blood samples preceding surgical removal of the primary tumor. The incidence of cancer cells in the blood of patients rapidly declined during the 48 hours postsurgery. The incidence of cancer cells in the blood remained stable in approximately 30% of patients to 14 days. The majority of breast cancer patients in this pilot study (even with small tumors and negative nodes) had detectable cancer cells in the blood prior to resection of the primary tumor. These findings justify further investigation. Successful application of this methodology may serve as a powerful indicator of which patients need systemic adjuvant therapy, the effectiveness of systemic adjuvant therapy, tumor recurrence, and early detection of breast cancer.