Angiogenesis is essential for the growth of solid tumors. We have observed previously that the vascular endothelial cells of astrocytic brain tumors express human telomerase reverse transcriptase (hTERT) mRNA, suggesting a role for telomerase in the angiogenesis of these neoplasms. Here, we used an in vitro model to demonstrate that the telomerase machinery might be trans-activated in primary endothelial cells by glioblastoma tumor cells. We found that glioblastoma cells in vitro do induce hTERT mRNA and hTERT protein expression, as well as telomerase enzyme activity in the endothelial cells, and that this phenomenon is mediated by diffusible factor(s). These results provide strong evidence of the involvement of telomerase in tumor angiogenesis and will stimulate research on antitelomerase drugs for treatment of malignant brain gliomas.