Obesity is associated with an increased frequency, morbidity, and mortality of several types of neoplastic diseases, including postmenopausal breast cancer. We found that human adipose tissue contains two populations of progenitors with cooperative roles in breast cancer. CD45(-)CD34(+)CD31(+)CD13(-)CCRL2(+) endothelial cells can generate mature endothelial cells and capillaries. Their cancer-promoting effect in the breast was limited in the absence of CD45(-)CD34(+)CD31(-)CD13(+)CD140b(+) mesenchymal progenitors/adipose stromal cells (ASC), which generated pericytes and were more efficient than endothelial cells in promoting local tumor growth. Both endothelial cells and ASCs induced epithelial-to-mesenchymal transition (EMT) gene expression in luminal breast cancer cells. Endothelial cells (but not ASCs) migrated to lymph nodes and to contralateral nascent breast cancer lesions where they generated new vessels. In vitro and in vivo, endothelial cells were more efficient than ASCs in promoting tumor migration and in inducing metastases. Granulocyte colony-stimulating factor (G-CSF) effectively mobilized endothelial cells (but not ASCs), and the addition of chemotherapy and/or of CXCR4 inhibitors did not increase endothelial cell or ASC blood mobilization. Our findings suggest that adipose tissue progenitor cells cooperate in driving progression and metastatic spread of breast cancer.