Reports of clinical studies of patients with knee disorders should routinely include their activity levels to enable comparison of treatment groups and to allow generalizability. The goal of this study was to develop and evaluate a new rating scale to measure activity levels of patients. We assessed reliability by administering the scale to 40 subjects on 2 separate occasions, 1 week apart. Validity was evaluated by comparing the activity rating on the new scale with that from other instruments that use activity level scales (concurrent construct validity) and also by correlating the score on the new scale with age (divergent validity). Patients easily understood the scale and were able to complete it in 1 minute. The reliability was high (intraclass correlation coefficient, 0.97). The scale also correlated well with existing activity rating scales: Spearman correlation coefficient for Cincinnati score, 0.67; for Tegner scale, 0.66; for Daniel scale, 0.52. The activity score was significantly inversely correlated with age (P = 0.002), indicating divergent validity. This instrument will facilitate generalizability of results and allow more accurate comparisons among patient groups in outcomes research in sports medicine.