QUESTION: Is an injury prevention program consisting of 10 exercises designed to improve stability, muscle strength, co-ordination, and flexibility of the trunk, hip and leg muscles (known as The11) cost effective in adult male amateur soccer players? DESIGN: Cost-effectiveness analysis of a cluster-randomised controlled trial. PARTICIPANTS: 479 adult male amateur soccer players aged 18-40 years. INTERVENTION: The intervention group was instructed to perform the exercises at each training session (2 to 3 sessions per week) during one soccer season. The exercises focus on core stability, eccentric training of thigh muscles, proprioceptive training, dynamic stabilisation, and plyometrics with straight leg alignment. The control group continued their usual warm-up. OUTCOME MEASURES: All injuries and costs associated with these injuries were compared between groups after bootstrapping (5000 replications). RESULTS: No significant differences in the proportion of injured players and injury rate were found between the two groups. Mean overall costs in the intervention group were €161 (SD 447) per athlete and €256 (SD 555) per injured athlete. Mean overall costs in the control group were €361 (SD 1529) per athlete and €606 (SD 1944) per injured athlete. Statistically significant cost differences in favour of the intervention group were found per player (mean difference €201, 95% CI 15 to 426) and per injured player (mean difference €350, 95% CI 51 to 733). CONCLUSIONS: The exercises failed to significantly reduce the number of injuries in male amateur soccer players within one season, but did significantly reduce injury-related costs. The cost savings might be the result of a preventive effect on knee injuries, which often have substantial costs due to lengthy rehabilitation and lost productivity. TRIAL REGISTRATION: NTR2416.