Sedentary behavior often begins in childhood and is associated with the development of risk factors for many chronic diseases in adulthood. Physical activity is considered important in the prevention of unfavorable changes in the risk factors. We investigated whether health-related behaviors, school type and health status are associated to physical activity among adolescents. A questionnaire was sent to all Finnish 16-year-old twins in 1991-93. A total of 3,254 twins responded. The response rate was 88%. Physical activity was classified into five categories (very active, active, moderately active, hardly active, inactive) based on self-reported frequency and intensity of physical activity. The analysis considered all subjects as individuals. Smoking was strongly associated with physical activity among girls and boys. Those who smoked regularly were less active. The type of school was also associated with physical activity. In general, those who attended comprehensive school or high school were physically more active, while those in vocational schools, particularly boys, were less active. Girls in lower physical activity groups reported more psychosomatic symptoms. Associations of self-reported health-related behaviors, school type and health status to physical activity seem to be the same among boys and girls. However, as the more active students are in comprehensive school or high school and the less active in vocational school, and physical inactivity is related to smoking and use of alcohol, health education should be tailored by school type.