<jats:title>Abstract</jats:title><jats:p>To investigate the possible role of oxidative RNA damage in the pathogenesis of Alzheimer's disease (AD), the concentrations of the oxidative stress marker 8‐hydroxyguanosine (8‐OHG) were measured in the cerebrospinal fluid (CSF) and the serum of patients with AD and control subjects. The concentration of 8‐OHG in CSF in AD patients was approximately fivefold that in controls (<jats:italic>P</jats:italic> < 0.001). The concentration of 8‐OHG in CSF decreased significantly with the duration of illness (r<jats:sub>s</jats:sub> = −0.48, <jats:italic>P</jats:italic> < 0.05) and the progression of cognitive dysfunctions (r<jats:sub>s</jats:sub> = 0.67, <jats:italic>P</jats:italic> < 0.01). However, the concentration of 8‐OHG in CSF showed no correlation with that in serum in both the controls and AD patients. In addition, the concentration of 8‐OHG in serum was not significantly altered in AD patients compared to that in controls, suggesting that the 8‐OHG concentrations in the CSF do not reflect those in serum and may be probably reflect those in brain tissue. These in vivo findings suggest a possible role of 8‐OHG and increased oxidative RNA damage in the early stage of the development of AD. © 2002 Wiley‐Liss, Inc.