OBJECTIVE: To compare the injury-related societal costs of an injury prevention program with usual warm up programs for amateur adult male soccer players. DESIGN: Cost effectiveness analysis of a cluster-randomized controlled trial. SETTING: Regional amateur male soccer competitions in the Netherlands during the 2009 season. Cost estimates in the Netherlands are made in 2009 Euros (&OV0556;1.00∼US$1.43 in late 2009). PARTICIPANTS: Soccer teams from 2 first-class competitions (the second-highest Dutch amateur level) were approached. Male players aged between 18 and 40 years, who were part of the first team at the start of the season, were eligible for inclusion. Twenty-three teams including 479 players were randomized and 456 (95%) were included in the analysis. INTERVENTION: The injury prevention program was The11, which includes 10 structured exercises developed by the FIFA Medical and Research Centre. The exercises, led by trained coaches, were designed to improve the stability, strength, co-ordination, and flexibility of the trunk, hip, and leg muscles and were performed 2 or 3 times per week as warm-up sessions. Compliance with the exercises was monitored randomly by the researchers. The control teams continued their usual warm-up routines. During the season, individual participants' exposure to training sessions or matches (in minutes) was reported weekly by the coaches. All participants completed questionnaires that included playing and injury history and current occupation and hours worked. Employment (vs studying) was nonsignificantly more common in the control group than the intervention group (68% vs 56%). MAIN COST AND OUTCOME MEASURES: Costs included those of the intervention, direct healthcare costs of injury, and indirect costs such as hours of work lost, which were recorded on a recovery form. Injuries occurring during the competition season were recorded weekly by the paramedical staff of the team. An injury was defined as a physical complaint sustained by a participant that resulted from a soccer training session or soccer match, whether or not there was lost time from soccer or need for medical attention. Full recovery was defined as participation throughout a training or match session. MAIN RESULTS: The players' injury rates were almost identical in the intervention and control groups (0.93 vs 0.94, representing 60.5% and 59.7% of players). The mean cost of The11, per player, was &OV0556;14 for the intervention group and &OV0556;0 for the control group. Direct healthcare costs per player were not significantly lower in the intervention group (difference, &OV0556;-44; 95% confidence interval [CI], -17 to 111). Indirect costs were lower in the intervention group (difference, &OV0556;-172; 95% CI, -352 to -28). The total cost per player was also lower in The11 group (difference, &OV0556;-201; 95% CI, -426 to -15). Direct health care costs per injured player were not significantly lower in the intervention group (difference, &OV0556;-76; 95% CI, -285 to 18). Indirect costs were lower in The11 group (difference, &OV0556;-288; 95% CI, -589 to -49). The total per injured player was, therefore, lower in The11 group (difference, &OV0556;-350; 95% CI, -733 to -51). CONCLUSIONS: The injury prevention strategy, The11, did not lower the rate of injuries in adult male soccer players, but the costs per player and per injured player were lower in the intervention group.