To determine if the relationship between habitual vigorous exercise and primary cardiac arrest (PCA) is modified by the presence of other risk factors, we investigated 133 male cases of PCA, 25-75 years of age, without prior heart disease or co-morbidity, and 133 controls, identified from the same community. Persons who did not engage in high-intensity leisure-time activity for more than 20 min per week were classified as non-vigorous. The risk of PCA was more than doubled for non-vigorous males, both in the presence and absence of other risk factors taken individually, i.e. age greater than or equal to 60, hypertension, cigarette smoking, obesity, and family history. The incidence of PCA attributable to lack of vigorous activity was greatest for older, hypertensive, or obese males; for each of these subgroups, it exceeded 19 cases/10,000 persons/yr. Efforts to discourage clinically healthy persons at risk of PCA from continuing to engage in vigorous exercise may be inappropriate.