Background. Recently antiangiogenic therapy with bevacizumab has shown a high but transient efficacy in glioblastoma (GBM). Indeed , GBM is one of the most angiogenic human tumors and endothelial proliferation is a hallmark of the disease. We therefore hypothesized that tumor cells may participate in endothelial proliferation of GBM. Materials and Methods. We used EGFR FISH Probe to detect EGFR amplification and anti-CD31 , CD105 , VE-cadherin , and vWF to identify endothelial cells. Endothelial and GBM cells were grown separately , labeled with GFP and DsRed lentiviruses , and then cocultured with or without contact. Results. In a subset of GBM tissues , we found that several tumor endothelial cells carry EGFR amplification , characteristic of GBM tumor cells. This observation was reproduced in vitro : when tumor stem cells derived from GBM were grown in the presence of human endothelial cells , a fraction of them acquired endothelial markers (CD31 , CD105 , VE-cadherin , and vWF). By transduction with GFP and DsRed expressing lentiviral vectors , we demonstrate that this phenomenon is due to cell fusion and not transdifferentiation. Conclusion. A fraction of GBM stem cells thus has the capacity to fuse with endothelial cells and the resulting hybrids may participate in tumor microvascular proliferation and in treatment resistance .