Members of the normally latent family of transcription factors signal/inducers and activators of transcription (Stat) are activated in a number of human tumors and tumor-derived cell lines. In the case of Stat3, it is believed that this activation leads to the induction of survival signals as well as increased proliferation. In this study, we demonstrate that Stat3 is constitutively activated in glioma and medulloblastoma tumors and that the activated protein localizes predominantly to the tumor endothelial cells in the highly vascularized glioma tumors. Our efforts to elucidate potential mechanism (s) for this activated protein have shown that coexpression of Stat3α and the vascular endothelial growth factor receptor-2 (VEGFR-2) result in ligand-independent activation of Stat3α tyrosine phosphorylation and subsequent transcriptional activation in non-endothelial cells. We also show that activated Stat3α can increase transcription from the vascular endothelial growth factor (VEGF) gene. Taken together, these results suggest that the activated Stat3α found in brain tumors may be due to the endothelial tyrosine kinase VEGFR-2 and that Stat3α may play a central role in autocrine VEGF activation.