There is a dearth of reliable and valid instrumentation that measures disability following injury and/or surgery of the knee joint that is responsive to clinically significant changes over time. The purpose of this investigation was to determine whether performance-based or patient-reported measures of function are more effective in estimating disability in individuals with an anterior-cruciate-ligament (ACL)-deficient knee. Subjective rating of knee function was used as the criterion measure for disability, and selected performance-based and patient-reported measures were used as estimation variables. Twenty-nine individuals with an ACL-deficient knee participated in this investigation. Step-wise regression analysis revealed that the Cincinnati Knee Scale, Lysholm Knee Scale, and hop index were the most effective estimates of disability. The results demonstrate that patient-reported measures are more related to the patient's level of disability in individuals with an ACL-deficient knee. More research is necessary to substantiate these findings.