Lymph node metastasis are the first prognostic factor in breast cancer diagnosis and an early event in metastatic spread. To assess the role of anti-apoptotic proteins in lymph node metastatic progression of human breast cancer cells we analyzed the metastatic activity of MDA-MB-435 cells transfected with the Bcl-x_L gene, after orthotopic inoculation in Nude Balb/c and in SCID mice. The luciferase gene was introduced by permanent transfection in the 435/Bcl-x_L and 435/Neo cells and used as a tumor marker to measure the number of tumor cells lodged in lymph nodes. We found that 435/Bcl-x_L tumor cells had enhanced organ-specific metastatic activity, preferentially lodging in peripheral lymph nodes, where at 45 days post-implantation we found 7 × 10⁶± 6× 10⁶ 435/Bcl-x_L.luc and 2 ±1.1 435/Neo.luc luciferase tagged tumor cell equivalents (TCEs). Metastases were abrogated in mice in which orthotopic tumors were induced with 435/Bcl-x_L-antisense cells. Additionally, in vitro experiments show that in 435 cells Bcl-x_L-antisense can override the emergence of resistance to apoptosis induced by TNF-α and TGF-β in cells overexpressing Bcl-x_L, increasing also adhesion to extracellular matrix proteins. These results point to the relevance of Bcl-x_L overexpression inducing lymph node metastasis of breast cancer cells, and to the value of this gene as a target for therapy in order to prevent metastasis.