Background: To translate and cross-culturally adapt the University of California at Los Angeles (UCLA) activity score into a simplified Chinese version (UCLA-C) and evaluate the reliability and validity of the UCLA-C for patients with both knee arthroscopy and total knee arthroplasty. Methods: Cross-cultural adaptation was performed according to the internationally recognized guidelines of the American Academy of Orthopaedic Surgeons Outcome Committee. A total of 200 participants (100 arthroscopy and 100 total knee arthroplasty) were recruited in this study. An intraclass correlation coefficient (ICC) was used to determine reliability. Construct validity was analyzed by evaluating the correlations between UCLA-C and the Tegner activity score, Knee Injury and Osteoarthritis Outcome Score, and the short-form (36) health survey. Results: The original version of the UCLA activity score was cross-culturally well adapted and translated into simplified Chinese. UCLA-C was found to have excellent reliability in both arthroscopy (ICC = 0.984, 95% confidence interval 0.976-0.989) and arthroplasty (ICC = 0.946, 95% confidence interval 0.920-0.964). Absolute reliability as evaluated by minimal detectable change was 0.789 and 0.837 for both arthroscopy and arthroplasty groups. Moderate to high correlations between UCLA-C and Tegner activity score (0.799, P <.001); Knee Injury and Osteoarthritis Outcome Score (0.449-0.715, P <.001); and Physical Functioning, Pain, General Health, and Social Functioning (0.549-0.746, P <.001) subdomains of short-form (36) health survey were observed. Conclusion: UCLA-C was demonstrated to have excellent acceptability, reliability, and validity in both arthroscopy and arthroplasty, and could be recommended for patients in mainland China. (C) 2017 Elsevier Inc. All rights reserved.