Sports leagues are traditionally considered natural monopolies. Understanding how they interact with one another is important. It is unclear how participation in a closed supra league affects competition in domestic leagues. This paper studies the effect of participating in basketball's Euroleague on team performance in national leagues. We find that Euroleague participation reduces the probability of victory by around 9% in ‘big’ leagues (i.e. those with the strongest teams). However, no significant effect was found in other leagues. A possible transmission channel is the reduction in the number of days between matches. We discuss the policy implications of this result.