Interleukin 10 (IL-10) was initially discovered on the basis of its ability to suppress cytokine synthesis. Additionally, it can exert immunosuppressive effects on a variety of cell types. Because patients with malignant gliomas present with a general impairment of the immune system, we investigated IL-10 expression in the glioma tissue. Because expression of IL-10 and IL-6 is associated in hematopoietic cells and IL-6 can act as an autocrine growth stimulator for glioblastoma cell lines, we looked in addition for a relationship between IL-10 and IL-6 expression. Using a quantitative reverse transcriptase polymerase chain reaction, IL-10 and IL-6 mRNA levels were determined in 37 glial tumors of different grades including 2 recurrencies, 3 specimens from normal brain tissue, and 3 glioblastoma cell lines. Expression of IL-10 mRNA was demonstrable in all tumors as well as in normal brain. High grade tumors and recurrent cases expressed significantly higher amounts of IL-10-specific mRNA compared with low grade tumors, whereas 2 of 3 cell lines showed only weak constitutive expression, mRNA for IL-6 was found in 86.5% of all gliomas with a correlation concerning the expression levels for both cytokines in 69% of gliomas. We suggest that IL-10 may contribute to the progression of astrocytomas by suppressing the patient's immune response, whereas IL-6 provides an additional growth advantage.