Neutrophils are traditionally thought of as terminal effectors of inflammatory reaction, but experimental studies suggest that they play a direct role in the inflammatory angiogenesis of tumors. Thus, further evidence in humans is required regarding the mechanisms by which neutrophils induce tumor angiogenesis. In this study, 4 cases of human gastric carcinomas with massive neutrophil infiltration were studied by light and electron microscopy, focusing on the inflammatory angiogenesis in the tumor stroma. At light microscopy, the tumors were advanced gastric carcinomas in which various degrees of tubular differentiation were present. Under an electron microscope, pericytes exhibited two major differentiated states with distinct ultrastructural features: a contractile phenotype and a synthetic phenotype. The contractile phenotype was characterized by abundant microfilaments. Synthetic pericytes contained abundant rough endoplasmic reticulum, lipid bodies, and numerous membrane-bound vesicles. These ultrastructural findings extend concept of contractile/synthetic phenotype modulation, originally described in smooth muscle cells, to tumor microvascular pericytes. Tumor microvasculature was also characterized by abortive or slit-like lumina, endothelial cell mitoses, and replicating basement membranes. These qualitative and observational transmission electron microscopy findings provide additional morphological evidence of active inflammatory angiogenesis in gastric carcinomas with massive neutrophil infiltration.