I nflammatory angiogenesis is a critical process in tumor progression and other diseases. The inflammatory cytokine IL-1β promotes angiogenesis, tumor growth, and metastasis, but its mechanisms remain unclear. We examined the association between IL-1β–induced angiogenesis and cell inflammation. IL-1β induced neovascularization in the mouse cornea at rates comparable to those of VEGF. Neutrophil infiltration occurred on day 2. Macrophage infiltration occurred on days 4 and 6. The anti–Gr-1 Ab-induced depletion of infiltrating neutrophils did not affect IL-1β– or VEGF-induced angiogenesis. The former was reduced in monocyte chemoattractant protein-1–deficient (MCP-1^–/–) mice compared with wild-type mice. After day 4, clodronate liposomes, which kill macrophages, reduced IL-1β–induced angiogenesis and partially inhibited VEGF-induced angiogenesis. Infiltrating macrophages near the IL-1β–induced neovasculature were COX-2 positive. Lewis lung carcinoma cells expressing IL-1β (LLC/IL-1β) developed neovasculature with macrophage infiltration and enhanced tumor growth in wild-type but not MCP-1^–/– mice. A COX-2 inhibitor reduced tumor growth, angiogenesis, and macrophage infiltration in LLC/IL-1β. Thus, macrophage involvement might be a prerequisite for IL-1β–induced neovascularization and tumor progression.