Tumor-associated macrophages (TAMs) have been implicated in promoting tumor growth and development. Here we present evidence that demonstrates that co-inoculation of male athymic nude mice with PC-3 prostate cancer cells and U937 promonocytic cells enhances tumor growth and increases tumor angiogenesis. Male athymic nude mice were co-inoculated with PC-3 and U937 cells (control or IL-4 stimulated) and tumor growth was monitored over time. Immunohistochemical analysis of tumor specimens was performed for proliferation markers (e.g., Ki67) and the effects of IL-4 stimulation on U937 cells were analyzed for chemokine expression. The presence of U937 cells increased the rate of tumor growth in vivo and stimulated increased microvascular density within the tumor bed. Stimulation of U937 cells with IL-4 resulted in a significant increase in several pro-angiogenic and pro-tumor chemokines (e.g., CCL2). Co-inoculation increases prostate cancer growth via upregulation of chemokines that induce angiogenesis within the tumor. J. Cell. Biochem. 103: 1–8, 2008. © 2007 Wiley-Liss, Inc.