Tumor stem cells are implicated in tumor initiation and maintenance. Recent studies have shown that a subpopulation of cells isolated from brain tumors can form neurospheres in vitro, and have multiple characteristic properties observed in neural stem cells. In vivo implantation of these cells can induce tumors that phenocopy original tumors, suggesting that tumor stem cells are involved in brain carcinogenesis. We found that a population of cells in human glioblastoma multiforme expressed multiple protein markers of neural stem cells including nestin, TUC-4, doublecortin and beta III-tubulin. In contrast, these markers were not expressed in human capillary hemangioblastoma or meningioma. Double immunolabeling showed that a portion of doublecortin-, beta III-tubulin-, TUC-4- and nestin-positive cells express Ki67 antigen, a cell proliferation marker. To investigate further whether these properties of tumor stem cells are correlated with their biological behavior, immunohistochemistry was performed on brain sections from astrocytomas of different grades using antibodies against neural stem cell markers. The number of cells expressing Ki67 antigen and neural stem cell markers was increased in relation to worsening histological grade of astrocytomas, indicating that the capacity for tumor stem cell proliferation may be clinically relevant. Thus, tumor stem cells in astrocytomas may be involved in carcinogenesis.