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PURPOSE: We studied the prognostic and predictive value of immunohistochemically detected occult tumor cells (OTCs) in lymph nodes and bone marrow aspirates obtained from node-negative breast cancer patients. All were classified as distant metastases-free using conventional staging methods. PATIENTS AND METHODS: A total of 484 patients with pT1-2N0M0 breast cancer and 70 with pT1-2N1M0 breast cancer and a single affected lymph node participated in our trial. Ipsilateral axillary lymph nodes and intraoperatively aspirated bone marrow were examined. All samples were examined for OTCs using monoclonal antibodies to cytokeratins 8, 18, 19. Immunohistological findings were correlated with other prognostic factors. The mean follow-up was 54 ± 24 months. RESULTS: OTCs were detected in 180 (37.2%) of 484 pT1-2N0M0 patients: in the bone marrow of 126 patients (26.0%), in the lymph nodes of 31 patients (6.4%), and in bone marrow and lymph nodes of 23 (4.8%) patients. Of the 70 patients with pT1-2N1MO breast cancer and a single involved lymph node, OTCs were identified in the bone marrow of 26 (37.1%). The ability to detect tumor cells increased with the following tumor features: larger size, poor differentiation, and higher proliferation. Tumors of patients with OTCs more frequently demonstrated lymph node invasion, blood vessel invasion, higher urokinase-type plasminogen activator levels, and increased PAI-1 concentrations. Patients with detected OTCs showed reduced disease-free survival (DFS) and overall survival (OAS) rates that were comparable to those observed in patients who had one positive lymph node. Multivariate analysis of prognostic factors revealed that OTCs, histological grading, and tumor size are significant predictors of DFS; OTCs and grading of OAS. CONCLUSION: OTCs detected by simultaneous immunohistochemical analysis of axillary lymph nodes and bone marrow demonstrate independent metastatic pathways. Although OTCs were significantly more frequent in patients with other unfavorable prognostic factors, they were confirmed as an independent prognostic factor for pT1-2N0M0, R0 breast cancer patients.