Tumor-associated macrophages have recently emerged as a key regulatory cell type during cancer progression, and have been found to promote tumor malignancy in the majority of studies performed to date. We show in this study that CD68+ macrophages positively correlate with tumor grade and liver metastasis in human pancreatic neuroendocrine tumors (PNETs). To investigate the potential mechanisms whereby macrophages can promote PNET progression, we crossed the RIP1-Tag2 (RT2) mouse model of pancreatic islet cancer to colonystimulating factor-1 (CSF-1)-deficientCsf1op/opmice, which have reduced numbers of tissue macrophages.Csf1op/opRT2 mice had a substantial reduction in cumulative tumor burden, which interestingly resulted from a significant decrease in angiogenic switching and tumor number, rather than an evident effect on tumor growth. In the tumors that did develop in CSF-1-deficient animals, however, there were no significant differences in tumor cell proliferation, apoptosis, angiogenesis or invasion. CSF-1 deficiency decreased macrophage infiltration by approximately 50% during all stages of RT2 tumor progression. Interestingly, several cytokines were upregulated in CSF-1-deficient RT2 tumors, and neutrophil infiltration was increased. These results show that macrophages are important for promoting PNET development and suggest that additional factors contribute to the recruitment and survival of myeloid cells in RT2 tumors in the absence of CSF-1.