1056 Background: The Forkhead Box protein M1 (FoxM1) is known to regulate a variety of biologic processes in mammalian cells including cell growth and survival, angiogenesis, DNA damage response, chemotherapeutic drug resistance, and cancer cell migration and invasion. We evaluate the role and significance of Fox M1 in primary breast cancer in vitro and analyzed the relation with FoxM1 expression and clinicopathologic features. Methods: Immunohistochemical staining was used for evaluation of cytoplasmic expression of FoxM1 with TMA of invasive breast cancer. In various breast cancer cell lines, we evaluated FoxM1 expression and treated docetaxel/cisplatin in combination with Siomycin A (FoxM1 inhibitor) for BT20 cell line. Results: From Nov 1995 to Jul 2007, in 84 patients with stage 1-3 invasive breast cancer, FoxM1 expression was noted in 58.7%. Median follow-up duration was 85.1 months. Lymphovascular invasion was positively correlated with FoxM1 expression (p=0.040). In multivariate analysis, FoxM1 expression (p=0.005), HR negativity (p=0.002), high histologic grade (p=0.023), hign nuclear grade (p=0.045), lymphovascular invasiveness (p=0.017), and stage 3 cancer (p=0.015) matched poor disease-free survival. In vitro study, FoxM1 was expressed BT474, JIMT-1, BT20, HCC-1937, and MDA-MB-231 cell lines. The inhibition of FoxM1 had synergistic effect on cisplatin treatment, not docetaxel in BT20 cell. Conclusions: FoxM1 expression was noted in triple negative breast cancer cell lines and its inhibition had synergistically cytotoxic effect on BT20 cell line in combination with cisplatin. Although the further in vivo and clinical study should be needed to draw the solid conclusions, FoxM1 could be both a promising target of treatment for triple negative breast cancer and a independent prognostic factor.