To test the hypothesis that oxidative stress is involved in breast cancer, we compared the levels of 8-hydroxy-2-deoxyguanosine (8-oxo-dG), an oxidized DNA base common in cells undergoing oxidative stress, in normal breast tissues from women with or without breast cancer. We found that breast cancer patients (N = 76) had a significantly higher level of 8-oxo-dG than control subjects (N = 49). The mean ( ± SD) values of 8-oxo-dG/105 dG, as measured by high-performance liquid chromatography electrochemical detection, were 10.7 ± 15.5 and 6.3 ± 6.8 for cases and controls, respectively (P = 0.035). This difference also was found by immunohistochemistry with double-fluorescence labeling and laser-scanning cytometry. The average ratios (×106) of the signal intensity of antibody staining to that of DNA content were 3.9 ± 7.2 and 1.1 ± 1.4 for cases (N = 57) and controls (N = 34), respectively (P = 0.008). There was no correlation between the ages of the study subjects and the levels of 8-oxo-dG. Cases also had a significantly higher level of 8-hydroxy-2-deoxyguanosine DNA glycosylase/apurinic lyase (hOGG1) protein expression in normal breast tissues than controls (P = 0.008). There was no significant correlation between hOGG1 expression and 8-oxo-dG. Polymorphism of the hOGG1 gene was very rare in this study population. The previously reported exon 1 polymorphism and two novel mutations of the hOGG1 gene were found in three of 168 cases and two of 55 controls. In conclusion, normal breast tissues from cancer patients had a significantly higher level of oxidative DNA damage. The elevated level of 8-oxo-dG in cancer patients was not related to age or to deficiency of the hOGG1 repair gene. © 2001 Wiley-Liss, Inc.