Spilleklar (The 11+) is a comprehensive warm-up programme developed to prevent injuries by improving strength, awareness and neuromuscular control during static and dynamic movements. Its effect was tested in a cluster randomised study of 13-17 year old female football players. For one football season the intervention teams used the complete exercise programme as their warm up for every training session and the running exercises as part of their warm up before every match (65 clubs, 1055 players), whereas the control teams continued to use their ordinary warm-up exercises (60 clubs, 837 players). The primary outcome was defined as an injury to the lower extremity (foot, ankle, lower leg, knee, thigh, groin, and hip). Exposure (participation in training and match), injuries, and to what extent the warm-up programme was carried out in each session (intervention clubs) was reported by coaches on weekly registration forms. A call center was established to record all injuries and contact coaches who were late on mailing the weekly registration forms. Every injured player was contacted by telephone to answer questions about the injury based on a standardised injury questionnaire. Although our primary outcome (an injury to the lower extremity) did not reach statistical significance, there was a significant reduction in several secondary outcome variables, including the rate of severe injuries, overuse injuries, and injuries overall. The risk of injury was reduced by one third and severe injuries by as much as one half among female footballers using the comprehensive warm-up programme.