Objective: It has been suggested that oxidative damage have been involved in the pathogenesis of neurodegenerative diseases. Although the role of oxidative stress in the etiology of these diseases is not clear, the increased levels of DNA damage and protein as well as DNA and mitochondrial oxidation have been demonstrated in patients with Parkinson's disease (PD) and Alzheimer's disease (AD) in a few studies. Material and Methods: In this study, the levels of DNA damage in the peripheral lymphocytes of patients with PD and multiple sclerosis (MS), and their age-matched healthy controls were determined by the comet assay with and without treatment of blood samples with 50μM hydrogen peroxide (H2O2) and the oxidative purine specific DNA repair enzyme, formamidopyrimidine glycosylase (Fpg). Results: The DNA damage in the lymphocytes of the patients with PD has been significantly increased compared to controls. However, no significant increase in the DNA strand breakage was detected in the patients with PD after treatment with either H2O2 or Fpg. Although the DNA damage in the MS patients was higher than their healthy controls, the difference was not found to be significant. A significant increase observed in the DNA damage after treatment with H2O2 and with Fpg in MS patients, have suggested the susceptibility to oxidative DNA damage in these patients. Conclusion: The data presented in this paper shows high level of DNA damage in the lymphocytes of patients with PD and MS which indicates the possibility of oxidative stres in these neurodegenerative diseases. © 2013 by Türkiye Klinikleri.