Glioblastoma patients are immunosuppressed, yet glioblastomas are highly infiltrated by monocytes/macrophages. Myeloid-derived suppressor cells (MDSC; immunosuppressive myeloid cells including monocytes) have been identified in other cancers and correlate with tumor burden. We hypothesized that glioblastoma exposure causes normal monocytes to assume an MDSC-like phenotype and that MDSC are increased in glioblastoma patients. Healthy donor human CD14(+) monocytes were cultured with human glioblastoma cell lines. Controls were cultured alone or with normal human astrocytes. After 48 hours, glioblastoma-conditioned monocytes (GCM) were purified using magnetic beads. GCM cytokine and costimulatory molecular expression, phagocytic ability, and ability to induce apoptosis in activated lymphocytes were assessed. The frequency of MDSC was assessed by flow cytometry in glioma patients' blood and in GCM in vitro. As predicted, GCM have immunosuppressive, MDSC-like features, including reduced CD14 (but not CD11b) expression, increased immunosuppressive interleukin-10, transforming growth factor-beta, and B7-H1 expression, decreased phagocytic ability, and increased ability to induce apoptosis in activated lymphocytes. Direct contact between monocytes and glioblastoma cells is necessary for complete induction of these effects. In keeping with our hypothesis, glioblastoma patients have increased circulating MDSC compared with normal donors and MDSC are increased in glioma-conditioned monocytes in vitro. To our knowledge, this has not been reported previously. Although further study is needed to directly characterize their origin and function in glioblastoma patients, these results suggest that MDSC may be an important contributor to systemic immunosuppression and can be modeled in vitro by GCM.