Markers of oxidative stress are increased in cerebrospinal fluid (CSF) of patients with Alzheimer disease (AD), although none of those reported are appropriate diagnostic markers because of the overlap between patients with AD and control subjects. To determine the ratio of 8-hydroxyguanine (8-OHG) levels in intact DNA to free 8-OHG in the ventricular CSF of patients with AD and age-matched control subjects. The most prominent marker of DNA oxidation is 8-OHG. Free 8-hydroxy-2'-deoxyguanosine (8-OHdG) was isolated from ventricular CSF taken at autopsy from 18 subjects with AD and 7 control subjects using solid-phase extraction columns. Levels were measured as the hydrolysis product, 8-OHG, using gas chromatography/mass spectrometry with selective ion monitoring. Intact DNA was isolated from the same CSF and the levels of 8-OHG were determined in the intact structures. Stable-labeled 8-OHG was used for quantification. A statistically significant (P<.05) 108-fold increase in the ratio of 8-OHG in intact DNA to free 8-OHG was observed in patients with AD. Analysis of the data distribution indicated that the lowest AD ratio was 3.5 times higher than the highest control ratio; there was no overlap of the 2 populations. Although the data for each individual measurement demonstrates overlap between patients with AD and control subjects, the ratio of 8-OHG intact in DNA to free 8-OHG demonstrates a delineation between patients with AD and control 8-OHG subjects and may be useful as a marker of disease progression or the efficacy of therapeutic antioxidant intervention.