Background: Arthritis is the leading cause of disability among adults in the United States affecting twenty-one million adults[1]. In addition, osteoarthritis is the second most costly chronic condition in the U.S[2]. Physical activity is a challenge in all patients and is associated with fewer functional limitations and lower risk for developing illness[3]. Currently, there are no objective measures of physical activity in advanced knee OA. Objectives: The purpose of this study was to quantify patient-reported changes in pain and function during the natural progression of osteoarthritis at 3, 6, and 9 months, and to correlate these metrics with objective activity monitors. Methods: 50 patients who were undergoing non-operative management of OA were enrolled. Patients were seen at baseline, 3 months, 6 months, and 9 months. At each visit, basic demographics and patient-reported measures (SF-36, WOMAC, and Charlson Co-morbidity index) were recorded. In addition, patients wore ActiGraph and activPal activity monitors for 7 days following the visit. Results: The average age of the enrolled participants was 57 with 82% of participants being less than 65 years of age. Most participants were female (64%), and 80% of participants had 1 or fewer medical co-morbidities on the Charlson Co-morbidity Index. Only 4% of patients were using assistive devices. The average WOMAC pain score was 68 and did not change from one time period to the next. The average SF-36 PCS score was 38 and the MCS was 54, and neither changed over time. The average SF-36 PCS score in patients with a WOMAC pain score less than 80 was 36, while in those with a WOMAC pain score greater than 80 it was 42.5. In contrast, analyses of the activPal found a decline in activity over the time period. In the first 19 patients wearing the activPal who were analyzed, 12 of 19 increased sedentary time at 9 months by an average of 18%. In addition, 15 of 19 participants decreased minutes of moderate to vigorous physical activity (MVPA) at 9 months by an average of 26%. Conclusions: In our study of 50 participants with osteoarthritis, patient-reported function did not change over a 9-month period. However, preliminary activity data suggests a decline. Further work will correlate patient-reported measures to the objective measures recorded by activity monitors to determine if objective monitors are preferable to detect early changes in activity due to OA. [1] (CDC), Centers for Disease Control and Prevention. Prevalence of arthritis—United States, 1997. MMWR Morb Mortal Wkly Rep 2001. May 4; 50:334-6. [2] Druss BG, Marcus SC, Olfson M, Pincus HA. The most expensive medical conditions in America. Health Affairs. 2002; 21:105-11. [3] Centers for Disease Control and Prevention (CDC). Physical activity among adults with a disability—United States, 2005. MMWR Morb Mortal Wkly Rep 2007. Oct 5;56(39):1021-4.