Using high-performance liquid chromatography prepurification/isotope dilution gas chromatography/mass spectrometry technique, we examined whether the amount of 8-hydroxyguanine and 8-hydroxy-2'-deoxyguanosine excreted into urine is higher in cancer patients with advanced-stage disease than in the control group. The control group consisted of 38 healthy subjects, and the patient group comprised 42 cancer patients suffering from metastasis of their primary tumors into the bones. We have found that the amount of the modified base (but not the nucleoside) excreted into urine is about 50% higher in cancer patients than in the control group. Because the presence of the modified base in urine may represent the primary repair product of oxidative DNA damage in vivo, our results suggest an important role of DNA glycosylases (most likely OGG1) in removal of the damage induced as a result of cancer development.