The associations of physical activity and cardiovascular fitness with cardiovascular disease risk factors were studied in 88 male adults, 180 female adults, 148 male children, and 142 female children. Subjects were families recruited from elementary schools in San Diego, California. Fitness (VO2 max) was measured by a submaximal cycle ergometer test. Physical activity was assessed by seven-day recall interview, yielding caloric expenditure, and by a simple self-rating of activity level. Risk factors included blood pressure, high density lipoprotein (HDL) cholesterol, the ratio of high density lipoproteins to low density lipoproteins (LDL), and body mass index. For all subgroups, fitness was strongly and significantly correlated with virtually all risk factors. After adjustment for body mass index, most fitness-risk factor associations were no longer significant. Seven-day caloric expenditure was significantly correlated with HDL/LDL only in female adults and children. The activity rating was significantly correlated with body mass index in all subgroups and with HDL/LDL in female adults and male adults. The simple activity rating tended to be correlated with fitness. The pattern of association was similar for adults and children.