Objective: We investigated the association between leisure time exercise participation and well-being (i.e., life satisfaction and happiness) and examined the causality underlying this association. Method: The association between exercise participation and well-being was assessed in around 8000 subjects, (age range 18-65 years) from The Netherlands Twin Registry (NTR). Causality was tested with the co-twin control method in 162 monozygotic (MZ) twin pairs, 174 dizygotic (DZ) twin and sibling pairs, and 2842 unrelated individuals. Results: Exercisers were more satisfied with their life and happier than non-exercisers at all ages. The odds ratio for life satisfaction given exercise participation was significantly higher than unity in unrelated pairs, and a trend was visible in DZ pairs. In MZ pairs, the odds ratio was close to unity. The pattern of odds ratios for happiness given exercise participation was similar. Conclusion: Exercise participation is associated with higher levels of life satisfaction and happiness. This association is non-causal and appears to be mediated by genetic factors that influence both exercise behavior and well-being. © 2006 Elsevier Inc. All rights reserved.