Inflammatory conditions are associated with tumor development. IL-1β is a multifunctional and proinflammatory cytokine that affects nearly all types of cells. To investigate the role of IL-1β in tumor growth in vivo, we transduced the retroviral vector coding human IL-1β gene into mouse Lewis lung carcinoma (LLC) cells and subsequently inoculated the transformant (LLC/IL-1β) to syngeneic C57BL/6 mice. Tumors derived from LLC/IL-1β grew faster (240%, day 18, vs null-vector control LLC/neo; p< 0.01) and showed more abundant vasculature (250%, vs LLC/neo; p< 0.05), whereas LLC/IL-1β cells, LLC/neo cells, and wild-type LLC cells did not show any significant difference in the growth rate in vitro. As compared with LLC/neo cells, LLC/IL-1β cells secreted 2-fold the amount of vascular endothelial growth factor and> 10-fold the amount of macrophage-inflammatory protein-2 (CXCL2), one of whose main functions is angiogenesis. Although LLC/IL-1β itself did not secrete hepatocyte growth factor (HGF), the tumor derived from LLC/IL-1β cells also contained a> 4-fold higher concentration of HGF, another angiogenic factor. In situ hybridization of HGF mRNA in LLC/IL-1β tumor sections demonstrated that stromal fibroblasts and infiltrating cells overexpressed HGF mRNA. Moreover, when cultured in the presence of HGF in vitro, LLC/IL-1β cells secreted even larger amounts of vascular endothelial growth factor and macrophage-inflammatory protein-2. The antiangiogenic agent TNP-470 and anti-CXCR2 Ab inhibited the tumor growth of LLC/IL-1β cells in vivo. These results indicated that secreting IL-1β into the tumor milieu induces several angiogenic factors from tumor and stromal cells and thus promotes tumor growth through hyperneovascularization.