Accumulation of DNA damage is implicated in aging. This is supported by the fact that inherited defects in DNA repair can cause accelerated aging of tissues. However, clear-cut evidence for DNA damage accumulation in old age is lacking. Numerous studies report measurement of DNA damage in nuclear and mitochondrial DNA from tissues of young and old organisms, with variable outcomes. Variability results from genetic differences between specimens or the instability of some DNA lesions. To control these variables and test the hypothesis that elderly organisms have more oxidative DNA damage than young organisms, we measured 8,5′-cyclopurine-2′-deoxynucleosides (cPu), which are relatively stable, in tissues of young and old wild-type and congenic progeroid mice. We found that cPu accumulate spontaneously in the nuclear DNA of wild-type mice with age and to a greater extent in DNA repair–deficient progeroid mice, with a similar tissue-specific pattern (liver > kidney > brain). These data, generated under conditions where genetic and environmental variables are controlled, provide strong evidence that DNA repair mechanisms are inadequate to clear endogenous lesions over the lifespan of mammals. The similar, although exaggerated, results obtained from progeroid, DNA repair–deficient mice and old normal mice support the conclusion that DNA damage accumulates with, and likely contributes to, aging.