Despite evidence demonstrating the role of β1-integrin in the regulation of cancer cell proliferation in vitro, the importance of this cell adhesion receptor during the initiation and progression of epithelial tumors in vivo remains unclear. Here we have used the Cre/LoxP1 recombination system to disrupt β1-integrin function in the mammary epithelium of a transgenic mouse model of human breast cancer. Using this approach, we show that β1-integrin expression is critical for the initiation of mammary tumorigenesis in vivo, and for maintaining the proliferative capacity of late-stage tumor cells. These observations provide a direct demonstration that β1-integrin plays a critical role in both the initiation and maintenance of mammary tumor growth in vivo.