657 Background: The presence of isolated tumor cells (ITC) in bone marrow (BM) in breast cancer patients has been evaluated in numerous studies. The majority of these studies have found correlation between the presence of ITC in BM and increased risk of relapse. In this study we assessed the association between the presence of BM and established prognostic factors in localizated breast cancer patients, and analyzed the influence of adjuvant therapies on the remove of these cells as a prognostic factor. Methods: BM aspirates from 296 patients with primary breast cancer with stages I-III (112 node negative, and 184 node positive), treated with adjuvant systemic chemotherapy (CT) with antracyclines, were evaluated. All patients underwent BM aspirate at diagnosis. In addition, 202 of these patients also underwent a second BM aspirate after CT. Carcinoma cells were detected using a standardized immunoassay with monoclonal antibody A45-B/B3 directed against cytokeratin. Detection of >= 2 tumor cells was scored as positive. Results: At diagnosis, 82 of 296 (28%) patients presented ITC in BM. The presence of ITC in BM did no correlate with standard prognostic variables such as lymph node status, tumor size, histological grade, hormonal receptor levels and expression of HER-2 neu. After a median follow up of 35 months (6–130), 53 patients (18%) relapsed (10 node negative, and 43 node positive). In the global group, the presence of ITC in BM at diagnosis showed a tendency to predict worse disease-free survival (DFS) and overall survival (OS). In the lymph node-positive group, 62% of patient showing ITC in the second BM aspirate after CT relapsed, whereas 21% of those with both negative BM aspirates presented relapse. The presence of ITC in the second BM aspirate was associated with inferior DFS (P=0,01) and OS (p=0,002). Conclusions: Our analytical approach allows the identification of micrometastatic BM disease in localized breast cancer patients that would be usually undetected using conventional methods. The persistence of occult metastatic cells in BM after adjuvant systemic therapy predicts relapse in localized breast cancer. No significant financial relationships to disclose.