To investigate angiogenesis during intestinal polyp development, we determined the microvessel density (MVD) in polyps of Apc knockout (Apc^Δ716) mice, a model for human familial adenomatous polyposis. We scored MVD also in several compound mutants carrying Apc^Δ716, namely, mice with an additional mutation in Smad4, in which the polyps progress into invasive adenocarcinomas; mice with a cyclooxygenase (COX)-2 gene (Ptgs2) mutation, in which adenoma growth is suppressed; and mice with prostaglandin E₂ EP receptor gene mutations. In both simple Apc^Δ716 and compound Apc^Δ716 Smad4 mutants, MVD increased in a polyp size-dependent manner only in the polyps expanded beyond a threshold of about 1 mm in diameter. These results indicate that tumor angiogenesis is stimulated only after tumors grow to a certain size, and this angiogenic switch is common to both benign adenomas and malignant adenocarcinomas. In Apc^Δ716 polyposis attenuated by the COX-2 gene mutation, in contrast, MVD did not increase even in polyps larger than 1 mm. The same phenomenon was observed in the compound mutant mice with Apc^Δ716 and the EP₂ receptor gene mutations, but not in other EP compound mutants. We also immunohistochemically studied COX-2 and angiogenic factors, vascular endothelial growth factor and basic fibroblast growth factor. Interestingly, expression of these proteins was also increased in polyps larger than 1 mm. These results suggest that, in both benign and malignant mouse intestinal tumors, stromal expression of COX-2 results in elevated prostaglandin E₂ levels that stimulate cell surface receptor EP₂, followed by induction of vascular endothelial growth factor that causes tumor angiogenesis.