['Circulating tumor cells (CTCs) correlate with worse prognosis in patients with metastatic breast cancer, but there are little data on CTCs in operable patients. We hypothesized that primary tumor characteristics would predict the likelihood of identifying CTCs in patients with operable breast cancer.', 'Clinical and pathological data from 92 patients with operable breast cancer were collected. The CellSearch system was used to detect CTCs in 30 ml of peripheral blood. CTCs were defined as nucleated cells lacking CD45 but expressing cytokeratins 8, 18, or 19. Univariate analysis was performed to determine if the presence of any primary tumor characteristic was predictive of CTCs. As a secondary objective we evaluated if nodal or bone marrow status was predictive of CTCs.', 'Thirty-eight percent of patients (35/92) had evidence of CTCs, with a median number of 1.0 CTC per CTC positive patient (range 1-22). HER2 status was the sole primary tumor characteristic that reliably predicted the presence of CTCs (P = 0.01, risk ratio = 3.66). No significant association was found between the presence of CTCs and tumor size (T), estrogen receptor (ER) status, progesterone receptor (PR) status, grade, histologic type, degree of nodal involvement (N), lymphovascular invasion (LVI) or Ki-67 proliferation. Bone marrow micrometastases were found in 17/64 (26.6%) of the patients but did not correlate with the presence of CTCs.'] Does hER2 status predict the presence of circulating tumor cells in patients with operable breast cancer?