Cross-talk between integrin receptors and activated growth factor receptors has been hypothesized to play a critical role in the initiation and progression of cancer. Despite in vitro evidence documenting the important role of integrin receptors in the regulation of cancer cell proliferation, the relative contribution of the integrin receptors to the initiation and progression of tumors remains unclear. Previous studies with a polyomavirus middle T mammary tumor model have indicated that targeted disruption of β1-integrin in the mammary glands of these mice completely blocks tumor induction. To further explore the general significance of these observations, we have crossed these conditional β1-integrin strains to a strain of mice carrying mouse mammary tumor virus/activated erbB2 (herein referred to as the NIC strain). In contrast to the tumor induction block in the polyomavirus middle T model, tumor onset in the β1-integrin–deficient NIC mice was delayed by only 30 d and was 100% penetrant. This modest effect on tumor induction was not a result of inefficient excision, as all tumors were confirmed as β1-integrin–null. Animals bearing β1-integrin–deficient ErbB2 tumors exhibited significantly reduced tumor volume, which was associated with increased tumor cell apoptosis and a reduction in tumor angiogenesis. In addition, β1-integrin–deficient tumors were compromised in their capacity to metastasize to the lung, a deficiency associated with abrogation of adhesion signaling. Taken together, these observations suggest that, although β1-integrin is dispensable for the initiation of ErbB2 tumor induction, it plays a critical role in metastatic phase of tumor progression.