Oxidative stress through the changes in the levels of reactive oxygen species and antioxidative parameters can cause various neurological disorders. The aim of the present study was to show antioxidant activity (AOA) and malondialdehyde (MDA) levels in affected people with Guillain-Barre syndrome (GBS) and multiple sclerosis (MS). A total of 15 GBS patients, 13 MS patients, and 15 age and sex matched controls were included in this study. MDA and AOA values were determined in both cerebrospinal fluid (CSF) and serum, spectrophotometrically. We have shown an increase in the values of MDA in the CSF of both GBS and MS patients (0.32 +/- 0.073 and 0.22 +/- 0.06 micromol/L) compared to the control (undetectable levels). Furthermore, a significant decrease in the serum MDA levels was shown in both GBS and MS patients (0.81 +/- 0.18 and 0.73 +/- 0.18 micromol/L) when compared to the control (1.7 +/- 0.46 micromol/L). A decrease was shown for serum AOA in both GBS (1.7 +/- 0.21 mmol/L) and MS patients (2.6 +/- 0.62 mmol/L) when compared to the control (3.2 +/- 0.17 mmol/L). However, a significant increase in the values of CSF AOA was shown in both MS and GBS patients (1.47 +/- 0.19 and 1.42 +/- 0.26 mmol/L) compared to the control (0.71 +/- 0.19 mmol/L). An imbalance between the levels of AOA and MDA in both CSF and serum can be followed in both MS and GBS patients.