OBJECTIVE: To investigate the effect of a structured warm-up programme designed to reduce the incidence of knee and ankle injuries in young people participating in sports. DESIGN: Cluster randomised controlled trial with clubs as the unit of randomisation. SETTING: 120 team handball clubs from central and eastern Norway (61 clubs in the intervention group, 59 in the control group) followed for one league season (eight months). PARTICIPANTS: 1837 players aged 15-17 years; 958 players (808 female and 150 male) in the intervention group; 879 players (778 female and 101 male) in the control group. INTERVENTION: A structured warm-up programme to improve running, cutting, and landing technique as well as neuromuscular control, balance, and strength. MAIN OUTCOME MEASURE: The rate of acute injuries to the knee or ankle. RESULTS: During the season, 129 acute knee or ankle injuries occurred, 81 injuries in the control group (0.9 (SE 0.09) injuries per 1000 player hours; 0.3 (SE 0.17) in training v 5.3 (SE 0.06) during matches) and 48 injuries in the intervention group (0.5 (SE 0.11) injuries per 1000 player hours; 0.2 (SE 0.18) in training v 2.5 (SE 0.06) during matches). Fewer injured players were in the intervention group than in the control group (46 (4.8%) v (76 (8.6%); relative risk intervention group v control group 0.53, 95% confidence interval 0.35 to 0.81). CONCLUSION: A structured programme of warm-up exercises can prevent knee and ankle injuries in young people playing sports. Preventive training should therefore be introduced as an integral part of youth sports programmes.