It has been shown that endothelial cells in solid tumors are cytogenetically abnormal. These cells are aneuploid with multiple chromosomes and multiple centrosomes. Unlike normal endothelial cells which remain diploid in long-term culture, the aneuploidy of tumor endothelial cells is exacerbated in culture suggesting that these cells are inherently unstable. It is speculated that this instability might compromise the effectiveness of antiangiogenesis therapy.