In order to study the incidence and mechanisms of injury in soccer and to recommend prophylactic measures, 180 players in a senior male soccer division were followed prospectively for 1 yr. Attendance records for games and practice sessions were kept, and all injuries were examined and treated by the same orthopaedic surgeon. One hundred twenty-four players incurred 256 injuries, mostly sprains and strains of the lower extremities. Of these, 62% were considered minor with ankle sprains being the most common (17%), while 11% were considered major with knee ligament sprains being the most frequent (32%). Overuse injuries were most frequent in the preseason training period. Traumatic leg injuries involved players with inadequate or no shin guards. Of the traumatic knee injuries, 11 of 18 (61%) occurred during a collision; non-contact knee injuries were frequently seen in those players with a history of knee injury and existing instability. Study of injury sequence disclosed that a minor injury was often followed within two months by a major one. In addition, with severe injuries incurred during fouls, the individual causing the penalty was injured. This prospective study suggested that those with knee instability and those allowed to resume play with poorly rehabilitated or clinically unhealed injuries are more apt to sustain further injury. Some injuries can be avoided by using better equipment and by observance of the rules.