Increased levels of oxidatively induced DNA damage have been reported in various cases of human pathogenesis like age-related and chronic diseases. Advances in experimental carcinogenesis associate high oxidative stress with genome instability and oncogenic transformation. Cancer biomarkers are helpful for early tumor diagnostics, prediction of tumor development, and analysis of individual tumors' response to therapy as well as recurrence. The repair resistant oxidatively induced clustered DNA lesions (OCDLs) could serve as a common indicator of oxidative stress in human malignant cells or tissues. To test this hypothesis, we assessed the levels of endogenous OCDLs in several human tumor and adjacent normal tissues from patients with liver, ovary, kidney, breast and colon cancer. These tumor tissues have already been shown to accumulate higher endogenous levels of gamma-H2AX foci. For the detection of clustered DNA lesions we used the human repair enzymes APE1, OGG1 and NTH1 as well as the Escherichia coli homologue Endonuclease III. In the majority of cases we detected higher levels of OCDLs in tumor vs. normal tissues but not always with a statistically significant difference and not with uniform tissue dependence. These data suggest for the first time the importance of endogenous non-DSB clusters in human cancer and their potential use as cancer biomarkers.