503 Background: The prognostic significance of CTCs in metastatic breast cancer has been demonstrated. If the presence of CTCs in early breast cancer would also predict an increased risk for relapse, this method might be used as an early marker for treatment efficacy and risk for disease recurrence. Methods: As part of the translational research project of the German SUCCESS-trial, we analyzed 23ml of peripheral blood from 1,500 N+ and high risk N- breast cancer pts before and after adjuvant taxane-based chemotherapy. The presence of CTCs was assessed with the CellSearchSystem (Veridex, USA). After immunomagnetic enrichment with an anti-Epcam-antibody, cells were labeled with anti-cytokeratin (8,18,19) and anti-CD45 antibodies to distinguish between epithelial cells and leukocytes. Results: In 10% of pts (n=143) >1 CTC was detected before the start of systemic treatment (mean 14, range 2–827). While we found 2 CTCs in 4% of pts, 3% had 3–5 CTCs and 1% 6–10 and >10 CTCs respectively. The presence of CTCs d...