A wide variety of oxidative DNA lesions are commonly present in untreated human and animal DNA. One of these lesions, 8-hydroxydeoxyguanosine, has been shown to lead to base mispairing (mutation) on DNA replication. Other lesions remain to be investigated in this respect. Oxidative DNA lesions on cell replication may, in appropriate circumstances, lead to proto-oncogene activation. Oxidative DNA damage, on fixation, may also lead to cytotoxicity followed by regenerative proliferation. The probable or possible importance of oxidative DNA damage is reviewed for various classes of carcinogens and natural processes, including metal ions, high-energy radiation, miscellaneous chemicals, tumor-promoting agents, polyhydroxyphenols/quinones, lipid metabolism, peroxisome proliferators and thyroid function. It is concluded that although the evidence needs considerable strengthening in many of these examples, the available information indicates the potential importance of oxidative DNA damage in the induction of tumors by these agents. It is also possible that non-cancerous degenerative diseases associated with aging are the result of the accumulation of lesions resulting from unrepaired oxidative DNA damage.