This randomized clinical study was designed to pro spectively determine the efficacy of a semirigid ankle stabilizer in reducing the frequency and severity of acute ankle injuries in basketball. Athletic shoe, playing surface, athlete-exposure, ankle injury history, and brace assignment were either statistically or experimen tally controlled. Participants in the study were 1601 United States Military Academy cadets with no prepar ticipation, clinical, functional, or radiographic evidence of ankle instability. Subjects experienced a total of 13,430 athlete-exposures in the 1990 and 1991 intra mural basketball seasons. Ankle injury was defined as acute trauma to the ankle ligaments that resulted in an athlete's inability to participate in basketball 1 day after the injury. Use of ankle stabilizers significantly reduced the frequency of ankle injuries. Reduction in ankle in juries, however, depended on the nature of injury (fewer contact injuries occurred among those who wore braces). Injury severity was not statistically reduced, and wearing the ankle stabilizer did not affect the fre quency of knee injuries. Attitude toward ankle stabilizer use improved as use of the stabilizer increased.