Five hundred and eighty high-school football players were studied over a period of two seasons to determine the effect of so-called prophylactic knee braces on the lower extremity. Two hundred and forty-seven athletes who wore single-hinged braces and eighty-three who wore double-hinged braces were paired for the same season of play with 250 athletes who were similar in height, weight, and playing position but who did not wear braces. The fifty-three injuries of the knee that occurred were significantly more frequent (p less than 0.001) in the group that wore single-hinged braces than in the matched, non-braced group. While there were more injuries of the knee in the participants who were double-hinged braces than in the matched controls, who did not wear braces, the increase in the number of injuries was not significant. There was also a dramatic increase (p less than 0.01) in the number of injuries of the ankle and foot in the athletes who wore braces. Our results question the efficacy of the braces that were studied and call attention to the potentially adverse effect of the braces on adjacent joints in the ipsilateral limb.