35 patients suffering from malignant stage III/IV glioma were enrolled into a prospectively randomized clinical trial. All patients were provided with standard oncologic treatment (neurosurgery, radiation, basic clinical care according to protocol and indication) and randomly divided into a) treatment group: receiving subcutaneous injections of a ML (a galactoside-specific lectin from mistletoe) standardized mistletoe extract, 1 ng ML-1/kg BW, twice a week for 3 months, starting on day 1 post surgery, b) control group: without additional complementary treatment. Immunophenotyping of peripheral blood leukocytes was done by flow cytometry (pre surgery; day 1, week 1, month 3 and 6 post surgery) to evaluate the immunomodulating capacity of ML-1 standardized mistletoe extract. Standard tumor destructive treatment of glioma proved to be suppressive for peripheral blood lymphocytes, since all subsets tested revealed statistically significant down regulation. Unlike the lymphocyte counts and activities of patients from the control group who gave preoperative values after 3-6 months), mistletoe treatment induced a statistically significant up regulation of cell counts (CD-3, CD4, CD-8 cells) and activities (CD-25, HLA/DR positive cells) after 3 months, as compared to preoperative values. Obviously, a strong immunoprotective/immunostimulatory effect was induced by the treatment of glioma patients with ML-1 standardized mistletoe extract which correlated with an improved quality of life, as determined by a standard questionnaire (Spitzer).