One hundred eighty players in a male soccer division were followed prospectively for 1 year to assess etiologic factors in soccer injuries. Range of motion and muscle strength in the lower extremities were measured in a preseason test. All injuries during 1 year were examined by the same orthopaedic surgeon. 42% of the injuries were considered to be due to player factors, such as joint instability, muscle tightness, inadequate rehabilitation, or lack of training. Ankle sprains were commoner in players with previous ankle sprains or clinical instability. 35% of the moderate (absence from practice greater than 1 week, less than 1 month) or major (absence greater than 1 month) injuries were preceded by minor (absence less than 1 week) injuries, reflecting inadequate rehabilitation. Players sustaining knee sprains not due to collision had reduced muscle strength in the injured leg. No other strength differences between injured and uninjured players were found. 63% of players had tight muscles. Strains more commonly affected players with muscle tightness. 71% of the injuries proved to be explicable by and associated with player factors, equipment, playing ground, or rules.