It is believed that families are important influences on the development of health habits, and the purpose of the present study was to examine the familial aggregation of physical activity. Physical activity habits were assessed by standardized interview in adults and children in 95 Anglo families and 111 Mexican-American families. The results indicated a moderate degree of aggregation of physical activity in both samples, and adjustment for body mass index was inconsequential. Intrafamily correlations tended to be higher in Mexican-Americans. Mother-child correlations usually were higher than father-child correlations. These findings support the hypothesis that the family is a significant influence on physical activity.