In order to test for the presence of familial aggregation in physical fitness and coronary heart disease risk factors, body fat, submaximal power output, muscular strength, muscular endurance, blood pressure, pulmonary functions, and several blood biochemical variables were measured in 304 nuclear families living in the Quebec city area. Analysis of variance indicated a larger between-family than within-family variation for all the variables. When all members of nuclear families were considered, intraclass correlations ranged from 0.21 to 0.34 (P less than or equal to 0.01). Interclass correlations computed for various pairs of relatives revealed significant parent-child and sibling correlations for all the variables (0.14 less than or equal to r less than or equal to 0.55; P less than or equal to 0.01). On the other hand, spousal correlations tended to be lower but significant (0.10 less than or equal to r less than or equal to 0.30; P less than or equal to 0.05) for all variables except subcutaneous fat and hemoglobin concentration. These results suggest that heredity and common lifestyle shared by members of nuclear families are responsible for the familial aggregation of physical fitness, coronary heart disease risk factors, and pulmonary functions. The findings also support the notion of considering the nuclear family as a unit of intervention in the application of preventive measures aimed at the reduction of several risk factors.