This article explores the pericytes on blood vessels and the endothelial sprouts in tumors. In mouse tumors, the pericytes (unlike the pericytes on normal vessels) uniformly express α-SMA and desmin even on capillary-size vessels. Pericytes form sleeves around endothelial sprouts that arise from tumor vessels, and the conclusion suggests that they may participate in blood vessel growth, and are a potential target in anti-angiogenic therapy.