Abstract : One of the leading etiologic hypotheses regarding Alzheimer's disease (AD) is the involvement of free radical-mediated oxidative stress in neuronal degeneration. Although several recent studies show an increase in levels of brain DNA oxidation in both aging and AD, there have been no studies of levels of markers of DNA oxidation in ventricular CSF. This is a study of levels of 8-hydroxy-2′-deoxyguanosine (8-OHdG), the predominant marker of oxidative DNA damage, in intact DNA and as the “free” repair product that results from repair mechanisms. Free 8-OHdG was isolated from CSF from nine AD and five age-matched control subjects using solidphase extraction columns and measured using gas chromatography/mass spectrometry with selective ion monitoring. Intact DNA was isolated from the same samples and the levels of 8-OHdG determined in the intact structures. Quantification of results was carried out using stable isotope-labeled 8-OHdG. By using this sensitive methodology, statistically significant elevations (p < 0.05) of 8-OHdG were observed in intact DNA in AD subjects compared with age-matched control subjects. In contrast, levels of free 8-OHdG, removed via repair mechanisms, were depleted significantly in AD samples (p < 0.05). Our results demonstrate an increase in unrepaired oxygen radical-mediated damage in AD DNA as evidenced by the increased presence of 8-OHdG in intact DNA and decreased concentrations of the free repair product. These data suggest that the brain in AD may be subject to the double insult of increased oxidative stress, as well as deficiencies in repair mechanisms responsible for removal of oxidized bases.