OBJECTIVE: To evaluate an in vivo pro-oxidant state in patients with ataxia telangiectasia (AT). METHODS: A set of oxidative stress endpoints were measured in 9 AT homozygotes, 16 AT heterozygotes (parents) and 83 controls (grouped in age ranges as for patients and parents, respectively). The following analytes were measured: (a) leukocyte 8-hydroxy-2'-deoxyguanosine (8-OHdG); (b) blood glutathione (GSSG and GSH); and (c) plasma levels of glyoxal (Glx) and methylglyoxal (MGlx). RESULTS: AT patients displayed a significant decrease in blood GSSG (p=0.012) and in MGlx plasma concentrations (p=0.012). A non-significant decrease in the GSSG:GSH ratio (p=0.1) and a non-significant increase in 8-OHdG and Glx levels were observed in AT patients vs. young controls (age range 4-35 years). AT heterozygotes failed to display any significant changes vs. adult controls (age range 36-68 years). CONCLUSION: No significant increase in oxidative stress biomarkers was detected in blood from AT patients. The decrease in GSSG and MGlx levels in AT patients may suggest an adaptive response to a pro-oxidant state in AT-related target organs.