The aim of the study was, to assess the influence of a knee brace on the development of anterior instability after anterior cruciate ligament rupture and to test aptitude for physical strain. 46 subjects with arthroscopically proven anterior cruciate ligament rupture were divided equally in a randomised order into two groups. Both groups carried out a standardized physical therapy program, while one group (0) received additional stabilization by means of a functional knee brace (SofTec, Bauerfeind). In addition, important features of the brace (stabilization capacity, safety perception and physical performance) were tested and compared to the non-braced contralateral leg among 23 healthy physical education students by means of a standardized scaled questionnaire (observational study). 0 showed lower (p < 0.05) development of the anterior instability by 46 % and lower (p < 0.05) reduction in circumference of the femur muscles by 25 %. Increasing the time interval between accident and beginning of the brace treatment, increased the difference of the therapeutical effect of the brace. In the observational study, the brace received a better evaluation regarding all test parameters. The reduction of developing anterior instability by means of a brace should facilitate - apart from an individual operation-time-arrangement - especially a conservative approach within the framework of anterior instability and physical activity. Aside from the improvement of the mechanical stabilization, indications for suitability in strain situations could be detected for the tested brace.