Background: Soluble CD40 ligand (sCD40L) is a powerful marker of cardiovascular risk. Exercise is known to decrease cardiovascular risk, but the impact of ultra-endurance exercise on sCD40L responses is unknown. Objective: To examine the relationship between ultraendurance exercise in trained athletes and levels of sCD40L and its natural ligand sCD40. Design: Control-trial, crossover design, exercise intervention study of sCD40L and sCD40 levels. Setting: Outdoor exercise and laboratory testing, single centre study, School of Physical Education, University of Otago, New Zealand. Participants: Nine trained ultra-endurance athletes. Interventions: Athletes exercised (cycled and jogged) for 17 of 24 h. Venous blood was sampled at baseline and serially throughout exercise and 24 and 48 h after exercise. The athletes completed a 24 h control trial on a separate occasion, in randomised order. Main outcome measurements: Mean levels of sCD40L and sCD40 during exercise and rest with 95% CIs. Results: sCD40L levels dropped steadily from baseline (median 4128 pg/ml) to a measured nadir at 24 h following exercise (median 1409 pg/ml) (p=0.01). The levels had started to rise again by 48 h after exercise. When measured as a group, sCD40L levels remained constant during a control rest period. sCD40 levels remained constant on both exercise and control days. Conclusion: Ultra-endurance exercise lowers the levels of the cardiovascular risk marker sCD40L in athletes. These results raise the possibility that exercise-induced changes in sCD40L may provide one of the mechanisms by which exercise lowers cardiovascular risk.