Malignant tumors induce angiogenesis and modulation of microvasculature. Based on histologic and immunohistochemical analysis of human surgical material, we describe here the occurrence of glomeruloid structures in gastrointestinal carcinomas, and compare them with the microvasculature in inflammatory granulation tissue. The glomeruloid structures were composed of clusters of mutually fused capillaries with prominent swelling of endothelial cells and pericytes. They were thought to be specific for glioblastoma of the brain. The glomeruloid structures were observed juxtaposed to carcinoma nests in one-third of gastric carcinoma of intestinal type and colorectal carcinoma in the area of invasive growth beyond the muscularis mucosae. They were not observed in gastric carcinoma of diffuse type, intramucosal carcinoma, or inflammatory granulation tissue. The glomeruloid structures can be regarded as an extreme example of endothelial hyperplastic changes observed in cancer stroma. Our results suggested that glomeruloid structures can occur in carcinomas as vascular reaction, a mechanism different from that in inflammatory granulation tissues.