A role for DNA damage is central to many theories of aging, but attempts to show an increase in DNA damage with age have yielded contradictory results. However, previous experiments have been of limited sensitivity, only able to examine induced (not basal) damage or pooled (not individual) cells. In this report, we apply a novel technique (Singh et al., 1988) to directly measure basal levels of DNA single-strand breaks and alkali-labile sites in individual human peripheral blood lymphocytes (PBL) obtained from young (less than 60 years) and old (more than 60 years) male donors. This approach shows that while average changes with age are small, changes in certain individuals and in certain cells may be large: the mean increase in damage was only 12%, but the increase in a subpopulation of highly damaged lymphocytes was 5-fold. However, most of this increase was contributed by just 3 of 17 older subjects. Further characterization of these individuals may shed light on the relationship between DNA damage and aging.