The levels of N-acetoxy-2-acetylaminofluorene (NA-AAF)-induced unscheduled DNA synthesis (UDS) were determined in resting mononuclear leukocytes from males in a geographically defined population. The mutagen sensitivity, as determined by the NA-AAF method, was associated with the risk indicators of cerebrocardiovascular disease (CCVD) rather than with CCVD itself. Thus, high blood pressure, age, and smoking--some of the most important risk indicators of CCVD--seem to be conditions which predispose an individual's mononuclear blood cells to increased likelihood of accumulating DNA damage. Hypertensives evidenced higher values of NA-AAF-induced UDS than did nonhypertensive controls, even though the former were treated in accordance with internationally accepted norms. We have previously shown that hypertensives with normalized blood pressure did not differ from controls, when assessed by the NA-AAF method. In the present study, the hypertensives' blood pressure values were 19/12 mm Hg higher than those of the controls, indicating a need for more intensive treatment. Borderline hypertensives did not differ from their controls when assessed by the NA-AAF method, even though their blood pressure values were 19/9 mm Hg higher than those of their controls. This indicates that the group of borderline hypertensives may be heterogeneous, and that some of the patients may be suffering from a condition different from that in those with established hypertension.