Data regarding oxidatively modified DNA bases suggest that cancer cells are more exposed to oxidative stress than adjacent non-tumorous tissue. This novel concept may contribute to the understanding of certain aspects of tumor biology such as activated transcription factors, genetic instability, chemotherapy-resistance and metastasis. We therefore tested this concept in human renal-cell carcinomas (RCCs) by evaluating the expression of hMTH1, an enzyme preventing the misincorporation into DNA of 8-oxo-dGTP (8-oxo-7,8-dihydrodeoxyguanosine triphosphate), an oxidized form of dGTP in the nucleotide pool. The expression of hMTH1 messenger RNA (mRNA) in RCC was significantly higher than that in adjacent non-tumorous kidney. Moreover, advanced-stage tumors showed significantly higher hMTH1 mRNA expression than early-stage tumors, and there was a modest linear correlation between hMTH1 expression and c-myc expression. The results provide logical support for the concept of "persistent oxidative stress in cancer" and suggest a role of hMTH1 mRNA level as a prognostic marker.