Although there are many published instruments designed to determine outcome following the treatment of knee injuries, only a few incorporate specific assessments of sports activity level and participation into the evaluation. This article reviews 3 of the most commonly used sports activity outcome instruments: the scales devised by Tegner and Lysholm, the Hospital for Special Surgery and the International Knee Documentation Committee. Problems and potential study biases that can arise with improper questionnaire design and data reduction techniques are reviewed, and recommendations are made to correct these problems. The problems identified include: (i) the failure to precisely define sports activity levels according to a specific sport and intensity of participation; (ii) the failure to sort populations according to overall intensity of athletic participation both before and after treatment; (iii) the failure to detect and sort from the population patients who return to sports and experience significant symptoms; (iv) the combination of work and sports activities into the same scale; and (v) the failure to detect alterations in sports participation caused by changes in lifestyle or non-knee-related factors. We have developed a sports assessment instrument, the Cincinnati Sports Activity Scale (CSAS). The methodology used to create this scale, its use in the assessment of return to or change in sports activities, and the assessment of specific functions of daily and sports activities, are briefly reviewed. The CSAS is based on 2 criteria: (i) the frequency of participation; and (ii) the general types of forces experienced by the lower extremity during the sport. The assessment of change in sports activities accounts for modifications in lifestyle and can also detect patients who have returned to sports but are experiencing significant symptoms and problems. Examples of data reduction and reporting are provided to represent practical situations from a prior investigation.