Tumor vasculature is known to express high levels of the longest splice variant of tumor endothelial marker 8 (TEM8). Little is known about its expression by tumor cells. Five of eight cell breast cancer cell lines tested expressed significant levels of the longest TEM8 splice variant (TEM8.1), and to a lesser extent, the shortest splice variant (TEM8.3). Breast cancer cell lines expressing high levels of TEM8 are known to be more invasive and typify a more aggressive basal-like phenotype. In vivo studies in the 4T1 murine model showed enhanced tumor growth associated with increased tumor vascularity and metastasis to lymph nodes and lungs. These data suggest that TEM8.1 expression in breast cancer cells confers a more aggressive, proangiogenic phenotype.