Intussusceptive microvascular growth refers to vascular network formation by insertion of interstitial tissue columns, called tissue pillars or posts, into the vascular lumen and subsequent growth of these columns, resulting in partitioning of the vessel lumen. While intussusception has been reported in normal developing organs, its existence in solid tumors has not been previously documented. By observing the growth of the human colon adenocarcinoma (LS174T) in vivo for a period of 6 weeks, we demonstrate that intussusception is an important mechanism of tumor angiogenesis. At the leading edge of the tumor, vascular growth was found to occur by both intussusception and endothelial sprouting. In the stabilized regions, intussusception led to network remodeling and occlusion of vascular segments. The formation of some tissue pillars appears to depend on intravascular blood-flow patterns or changes in intravascular shear stress. The rapid vascular remodeling by intussusception could possibly contribute to intermittent blood flow in tumors.