STUDY OBJECTIVE: To examine the associations between frequency of leisure time exercise and cardiovascular risk factors in adolescents. DESIGN: Cross sectional survey carried out over a 3 month period in 1984. SETTING: All 24 schools in North Karelia province and 16 randomly selected schools in Kuopio province. PARTICIPANTS: A total of 1142 boys and girls aged 15, randomly selected from 40 schools (16 boys and 16 girls from each), participated out of a possible total sample of 1280. Main reason for non-participation was absence from school but a small number refused to participate. MEASUREMENTS AND MAIN RESULTS: The investigation comprised a medical examination, including anthropometry, a self administered questionnaire checked by interview, a parental questionnaire, and a blood sample for biochemical estimations. The main findings were: (1) Leisure time exercise was inversely related to daily smoking (Spearman's rho, boys -0.16, p less than 0.01; girls -0.13, p less than 0.01) but was not related to serum lipoproteins or blood pressure. The inverse association between exercise and smoking was independent of socioeconomic family background. (2) Body mass index and sexual maturation were associated with systolic blood pressure, and among boys they were inversely related to high density lipoprotein cholesterol. (3) The lack of linear trends between exercise and biological risk factors may be explained by the high prevalence of leisure time exercise in the sample (72% of boys and 71% of girls exercised at least 2-3 times per week, in addition to physical education classes at school). CONCLUSIONS: Among 15-year-old eastern Finnish boys and girls, leisure time exercise is favourably associated with the main behavioural cardiovascular risk factor, smoking, but not with serum total cholesterol or blood pressure.