The ratio between oxidized (GSH) and reduced (GSSG) glutathione is considered to be a marker of oxidative damage of an organism. Values of GSH/GSSG below 10 are signal of increased oxidative stress influencing the human organism due to the progressing disease or as a reaction on applied treatment. In our experiment, the GSH/GSSG was determined in patients with brain and prostate carcinoma by high performance liquid chromatography with electrochemical detection. Samples from three groups of patients suffering from brain cancer (Ependymoma anaplastic and medulloblastoma) and prostate cancer were analyzed using optimized method for determination of GSH and GSSG by HPLC-ED. The method recovery of 86-93% was determined and from obtained concentrations the GSH/GSSG ratio was established. The lowest ratio (average of 1.4) was determined in the group (n=4) of patients suffering from Ependymoma anaplastic. Patients (n=15) diagnosed with medulloblastoma the average ratio of 3.5 was determined. Prostate cancer patients exhibit the average ratio of 3.9. In two control groups of healthy persons, the GSH/GSSG of 11.6 and 10.8 was determined. It can be concluded from the results obtained that different tumor diseases have different impact on the redox status of patients.