Recently tau immunoreactivity has been observed in astrocytes in Alzheimer's disease and other neurological diseases. We examined the immunohistochemical localization of tau in 110 human brain tumors. Tau was widely distributed in the glial neoplastic cells and the reactive astrocytes in tumor tissues. In human surgical specimens positive immunostaining for tau was frequently observed in astrocytic tumors, oligodendroglial tumors, and glioblastoma, as well as neuronal tumors. The astrocytic neoplastic cells in medulloblastoma and other poorly differentiated tumors were also stained. In contrast, no immunoreactivity was observed in meningiomas and schwannomas. The expression of tau in brain tumors was mainly restricted to those cells with astrocytic features rather than small immature cells. The expression of tau mRNA was also demonstrated in astrocytic tumors. In conjunction with the findings of tau-positive astrocytes in some degenerative disorders, astrocytes are considered to have a potential to express tau through neoplastic transformation and reactive processes.