BACKGROUND: Ankle injuries account for 30 to 60% of all parachuting injuries. This study was designed to determine if outside-the-boot ankle braces could reduce ankle sprains during Army paratrooper training. METHODS: The randomized trial involved 777 volunteers from the U.S. Army Airborne School, Fort Benning, Ga. Of this group, 745 completed all study requirements (369 brace-wearers and 376 non-brace-wearers). Each volunteer made five parachute jumps, for a total of 3,674 jumps. RESULTS: The incidence of inversion ankle sprains was 1.9% in non-brace-wearers and 0.3% in brace-wearers (risk ratio, 6.9; p = 0.04). Other injuries appeared unaffected by the brace. Overall, 5.3% of the non-brace group and 4.6% of the brace group experienced at least one injury. The risk ratio for injured individuals was 1.2:1 (non-brace to brace groups; p = 0.65). CONCLUSION: Inversion ankle sprains during parachute training can be significantly reduced by using an outside-the-boot ankle brace, with no increase in risk for other injuries.