The expression of thrombospondin‐1 (TSP‐1) and its role in gliomas have not been well examined. In the present study TSP‐1 expression in a panel of malignant glioma cell lines and the expression of TSP‐1 and transforming growth factor (TGF‐β) proteins in low‐grade and malignant glioma tissues were investigated. Reverse transcription‐polymerase chain reaction analysis showed that nine of nine malignant glioma cell lines expressed TSP‐1 mRNA, and seven of nine glioma lines expressed TSP‐2 mRNA. Production and secretion of TSP‐1 were examined in the T98G glioblastoma cell line by western blot analysis. Total TSP‐1 protein content in the supernatant was 10 times higher than that in the cell lysate. Secretion of TSP‐1 was examined in these glioma cell lines by western blot analysis. All glioma lines secreted significant levels of TSP‐1. Bioassay showed that all tumor lines had the capacity to activate latent TGF‐β. Localization of TSP‐1, TGF‐β1, ‐β2, and ‐β3 was examined immunohistochemically in surgically resected glioma tissues, including 11 glioblastomas, six anaplastic astrocytomas, and eight astrocytomas. Most glioblastomas expressed high levels of both TSP‐1 and TGF‐β. Anaplastic astrocytomas expressed moderate levels of TSP‐1 and TGF‐β. Most malignant gliomas expressed various levels of TGF‐β1, ‐β2, and ‐β3. The expression of both proteins, however, was weak in low‐grade gliomas. Normal brain tissues around the tumors were negatively or very weakly positively stained for TSP‐1 and TGF‐β. These results indicate that most malignant glioma cells express TSP‐1 in vitro and in vivo, and the expression of TSP‐1 and TGF‐βin vivo correlates with the histologic malignancy of glioma. Overexpression of both TSP‐1 and TGF‐β may increase the biologic malignancy of malignant gliomas, through generating the active form of TGF‐β in tumor tissues.