Objectives: The study aimed to gain a better understanding of the relationship between leisure time physical activity and smoking in adolescence by investigating adolescents' motives for participation in leisure time physical activity. Methods: The study involved cross-sectional and longitudinal data from a postal survey involving 16-22-year old Danes. The hypothesized associations were examined using hierarchical logistic regression analyses. Results: An inverse association between participating in leisure time physical activity and smoking was found. Participation in leisure time physical activity for friendship or competition reasons were conditions that strengthened the inverse association between physical activity and smoking in males. In contrast, participation for the reason of losing weight or gaining self-esteem appeared to weaken the inverse association among females, In addition, the motives enjoyment, health and, in females, friendships and stress relief were associated with less smoking irrespective of participation level, while the motives self-esteem, losing weight and, in males, friendships were unrelated or even positively related to smoking. Conclusions: The association between adolescents' leisure time physical activity and smoking behavior differs with the underlying motivation for the activity. (C) 2009 Elsevier Ltd. All rights reserved.