To clarify whether the neovasculature of brain tumors preserves blood-brain barrier (BBB) functions, we studied the expression of a tight junction-related protein, Zo-1, using immunohistochemistry. Twenty-six astrocytic tumors were examined using an anti-Zo-1 Mab, and Zo-1 expression was compared with the expression of vasicular endothelial growth factor (VEGF), vascular endothelial growth factor receptor (VEGFR) (fit-1), and antiproliferative cell nuclear antigen (PCNA). A positive reaction for Zo-1 was seen in the endothelial cells in micro-blood vessels in all astrocytic tumors. The reactions for Zo-1 in the endothelial cells forming glomeruloid proliferations in newly formed micro-blood vessels in high-grade tumors were weaker than those in the endothelial cells of normal cerebral capillaries. Although there is a negative correlation between positive immunoreactions for BBB-related proteins and the expression of VEGF of the endothelial cells in micro-blood vessels, the proliferative activity of tumor cells, and histological grades, the present findings suggest that the endothelial cells of the neovasculature of high-grade tumors preserve partial BBB function at the cellular level. Because of the ease of immunohistochemical procedures compared with electron microscopic examination, the immunohistochemical detection of Zo-1 should provide a useful marker for tight junctions.