The purpose of this prospective, randomized study was to determine the efficacy of a prophylactic knee brace to reduce the frequency and severity of acute knee injuries in football in an athletic environment in which the athletic shoe, playing surface, athlete-exposure, knee injury history, and brace assignment were either statistically or experimentally controlled. The participants in the study were 1396 cadets at the United States Military Academy, West Point, New York, who experienced a total of 21,570 athlete-exposures in the 1986 and 1987 fall intramural tackle football seasons. The use of prophylactic knee braces significantly reduced the frequency of knee injuries, both in the total number of subjects injured and in the total number of medial collateral ligament injuries incurred. However, the reduction in the frequency of knee injuries (total and medial collateral ligament) was dependent on player position. Defensive players who wore prophylactic knee braces had statistically fewer knee injuries than players who served as controls. This was not true of offensive players who served as controls; they had statistically no difference in the number of knee injuries from players who wore prophylactic knee braces. The severity of medical collateral ligament and anterior cruciate ligament knee injuries was not significantly reduced with the use of prophylactic knee braces.