Abstract Non-small cell lung cancer (NSCLC) accounts for 80-85% of all cases of lung cancer and is known to be caused by smoking and other carcinogenic agents. Oxidatively induced DNA damage is involved in multiple modifications and mutations in DNA, thereby in pathogenesis of cancer as well as other diseases. In this study, we compared the tumor tissues to the surrounding healthy tissues obtained from 29 NSCLC patients who had undergone surgical resection. The levels of oxidatively induced DNA lesions such as 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyGua), 8-hydroxyguanine (8-OH-Gua) and (5′S)-8,5′-cyclo-2′-deoxyadenosine (S-cdA) were measured by gas chromatography/isotope-dilution mass spectrometry and liquid chromatography/isotope-dilution mass spectrometry. We found that the level of S-cdA was statistically significantly greater in cancer tissues than that in control tissues (p=0.0063), whereas the levels of FapyGua and 8-OH-Gua showed no significant difference between the two groups. There was no significant correlation between DNA damage and age, smoking or pathological stage. These results provide evidence for significant accumulation of S-cdA in tumor tissues compared to surrounding healthy tissues in NSCLC. The evaluation of the alterations in the levels of DNA lesions may contribute to the elucidation of the DNA repair defects in NSCLC. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3943.