A family history of heart attack is reported to be an independent predictor of cardiovascular death in men. In a 9 year follow-up of 4014 adults from 40 to 79 years old in the Rancho Bemardo Study, men under 60 years of age with a family history of heart attack were at fivefold increased risk. In this study, we sought to determine whether modifiable risk factors, i.e., blood pressure, plasma cholesterol, obesity, and cigarette smoking, have a differential effect on cardiovascular risk in those with and without a family history of heart attack. For both sexes, cigarette smoking was a stronger predictor of cardiovascular disease in those with a family history of heart attack (relative risk of smokers vs nonsmokers was 2.5 for men and 4.0 for women) than in those with no such family history (relative risk of smokers vs nonsmokers was 1.1 for men and 1.7 for women). Conversely, an increased risk of cardiovascular mortality in men with a family history of heart attack was present predominantly in smokers (relative risk related to positive family history was 1.2 in nonsmokers, and 3.3 in smokers). An estimated 68% of the excess deaths in men with a family history of heart attack were attributable solely to the interaction of family history with smoking habit and were therefore potentially avoidable. The risk of cardiovascular disease associated with an apparently inherited predisposition appears to be profoundly affected by modifiable behavior.