As a part of a larger research programme called Cardiovascular Risks in Young Finns, this longitudinal study is concerned with physical activity and participation in, and dropout from, sport among children and adolescents in Finland, the interrelationship between participation in sports and family and living environment and the impact of the physical activity experienced in childhood and adolescence on later interest in physical activity. The data were gathered in 1980 from 3596 boys and girls who were 3, 6, 9, 12, 15 and 18 years of age. The measurements were carried out in 1983, 1986 and 1989. The present study will be concerned with the 9‐year‐old and older subjects only. Physical activity was measured by means of a questionnaire. It is at its peak at the age of 12, after which it is reduced considerably, but the intensity and the strain are increased at the same time. Among young women (24 and 27 years of age), physical activity is more common than it is among men. Girls have stepped up their participation in organized sport during the past decade. Physical activity during youth is a significant but weak predictor of the physical activity that takes place 9 years later. The best predictors are the school grade for physical education and participation in organized sport. Children's physical activity correlates with the social status of the family and the parents' interest in physical activity as well as the environment in which the subjects l