OBJECTIVE: To determine the validity, reliability, and sensitivity of a published chronic pain index by completion of a questionnaire in Finnish by owners of dogs with chronic signs of pain caused by osteoarthritis. ANIMALS: 61 client-owned dogs with osteoarthritis. PROCEDURES: Validity, internal consistency, and repeatability testing of the questionnaire were evaluated by completion of questions in Finnish by owners of 61 dogs; the questionnaire was named the Helsinki chronic pain index (HCPI). Sensitivity testing of the questionnaire was determined from data of 2 smaller groups of dogs treated with carprofen (n = 17) or placebo (17). Owners completed the questionnaire 5 times during a 16-week period. Psychometric properties of the HCPI were evaluated. RESULTS: Principal component analysis yielded only a single stable component, indicating that the HCPI was best explained as an 11-item single construct index of chronic pain. Changes in the HCPI correlated well with change in quality of life and with change in the mobility visual analogue scale (r = 0.72 and r = 0.67, respectively), indicating a high predictive validity. Internal consistency (Cronbach A value = 0.82) and test-retest reliability (intraclass correlation coefficient = 0.84 for items and 0.92 for the HCPI) were high. Changes in scores (from baseline to treatment and back to no treatment) between the carprofen treatment group and placebo control group were significant, indicating high sensitivity. CONCLUSIONS AND CLINICAL RELEVANCE: The Finnish version of the HCPI provided a valid, reliable, and responsive tool for assessment of response to treatment in dogs with osteoarthritis.