OBJECTIVES: To determine 1) whether scores of functional tests improve in newly diagnosed dogs with osteoarthritis(OA) administered Carprofen (Rimadyl® Chewable Tablets, Zoetis, Inc., Parsippany, NJ); and 2) whether longer administration provides additional improvement. METHODS: Newly diagnosed dogs with OA (n=133) given labelled dose for between 30 and 120 days. Five activities (walking, jogging, sitting/lying, walking up and down stairs) were video-recorded (n=399) on days 0, 30 (n=82-122) and 120 (n=14-38). Ability scores were recorded by an independent investigator blinded to timepoint using a 100mm visual analogue scale (VAS). Associations between time and scores were tested using linear mixed models fitted in SAS (SAS 9.4, SAS Institute Inc., Cary, NC). RESULTS: Duration of treatment had a significant positive effect on improvement for all activities (walk, P = 0.005; jog, P = 0.006; sitting/lying, P< 0.001; walking upstairs, P = 0.003; walking down stairs, P = 0.03). Walking scores significantly improved between days 0 and 30 (P = 0.004). Jogging scores significantly improved between days 0 and 30 (P = 0.004). Sitting or lying down scores significantly improved between days 0 and 30 (P< 0.0001) and between days 0 and 120 (P = 0.0002). Ability to walk upstairs significantly improved between days 0 and 120 (P = 0.003) and between days 30 and 120 (P = 0.025). Ability to walk down stairs significantly improved between days 0 and 30 (P = 0.023). CONCLUSIONS: Activities of daily living are improved by the administration of carprofen in dogs with clinically diagnosed OA, and a longer duration of treatment is associated with additional improvement.