Etk/Bmx, a member of the Tec family of nonreceptor protein-tyrosine kinases, is characterized by an N-terminal pleckstrin homology domain and has been shown to be a downstream effector of phosphatidylinositol 3-kinase. P21-activated kinase 1 (Pak1), another well characterized effector of phosphatidylinositol 3-kinase, has been implicated in the progression of breast cancer cells. In this study, we characterized the role of Etk in mammary development and tumorigenesis and explored the functional interactions between Etk and Pak1. We report that Etk expression is developmentally regulated in the mammary gland. Using transient transfection, coimmunoprecipitation and glutathione S-transferase-pull down assays, we showed that Etk directly associates with Pak1 via its N-terminal pleckstrin homology domain and also phosphorylates Pak1 on tyrosine residues. The expression of wild-type Etk in a non-invasive human breast cancer MCF-7 cells significantly increased proliferation and anchorage-independent growth of epithelial cancer cells. Conversely, expression of kinase-inactive mutant Etk-KQ suppressed the proliferation, anchorage-independent growth, and tumorigenicity of human breast cancer MDA-MB435 cells. These results indicate that Pak1 is a target of Etk and that Etk controls the proliferation as well as the anchorage-independent and tumorigenic growth of mammary epithelial cancer cells.