During the last decade the importance of reactive oxygen species as major contributors to various types of cancer, heart diseases, cataracts, Parkinson's and other degenerative diseases that come with age, and to natural aging has become apparent. Mitochondria are the most important intracellular source of reactive oxygen. Mitochondrial DNA is heavily damaged by reactive oxygen at the bases, as indicated by the high steady-state level of 8-hydroxydeoxyguanosine, the presence of which causes mispairing and point mutations. Mitochondrial DNA is also oxidatively fragmented to a certain extent. Conceivably, such fragmentation relates to deletions found in mitochondrial DNA. Point mutations and deletions have recently been shown to be etiologically linked to several human diseases and natural aging. Future studies should address the causal relationship between mitochondrial dysfunction, production of reactive oxygen species, and aging.