Colony-stimulating factor-1 (CSF-1) and its receptor (CSF-1R) have been implicated in the pathogenesis and progression of various types of cancer, including breast cancer. This is based on high levels of circulating CSF-1 in patient sera with aggressive disease and increased CSF-1R staining in the tumor tissues. However, there have been no direct in vivo studies to determine whether a CSF-1 autocrine signaling loop functions in human breast cancer cells in vivo and whether it contributes to invasion. Recently, in mouse and rat models, it has been shown that invasion and metastasis are driven by an epidermal growth factor (EGF)/CSF-1 paracrine loop between tumor cells and host macrophages. In this macrophage-dependent invasion, tumor cells secrete CSF-1 and sense EGF, whereas the macrophages secrete EGF and sense CSF-1. Here, we test the hypothesis that in human breast tumors, the expression of both the CSF-1 ligand and its receptor in tumor cells leads to a CSF-1/CSF-1R autocrine loop which contributes to the aggressive phenotype of human breast tumors. Using MDA-MB-231 cell-derived mammary tumors in severe combined immunodeficiency mice, we show here for the first time in vivo that invasion in a human mammary tumor model is dependent on both paracrine signaling with host macrophages as well as autocrine signaling involving the tumor cells themselves. In particular, we show that the autocrine contribution to invasion is specifically amplified in vivo through a tumor microenvironment-induced upregulation of CSF-1R expression via the transforming growth factor-beta1.