Estrogen is important for breast carcinogenesis and the majority of breast cancers maintain hormone dependency. Estrogen has the ability to stimulate both breast epithelial cell growth and angiogenesis, and a well-characterized in vivo cancer model where these functional interactions can be studied is lacking. We demonstrate estrogen dependent angiogenesis, growth in vivo, and proliferation in vitro, in explants from polyoma middle T transgenic mouse mammary tumors. Thus, in addition to genetic similarities, this model also exerts a sex hormone, and angiogenic phenotype similar to human breast cancer. This immune-competent animal model offers the opportunity to study molecular events in estrogen dependent breast cancer.