Objective: Assess the effects of tramadol ER once daily versus placebo in patients with moderate to moderately severe chronic pain due to osteoarthritis of the knee or hip. Methods: Data for this post-hoc analysis were from a 12-week, randomized, double-blind, placebo-controlled, once-daily fixed dose-study of tramadol ER (100mg-300mg). Patients completed the WOMAC questionnaire at baseline and weeks 1, 2, 3, 6, 9 and 12. Items in each WOMAC subscale – pain (5-items), physical functioning (17-items) and stiffness (2-items) were combined and normalized from 0-to 100. The minimum clinically important difference (MCID) set at ten points improvement was determined from the literature. Mean subscale scores, percent mean change from baseline and the proportion of patients achieving a MCID at week 1 and 12 were assessed. Results: A total of 809 patients were analyzed (604-tramadol ER; 205-placebo). Both cohorts had similar demographic and clinical characteristics at baseline. At week 1, mean change in WOMAC global and subscale scores from baseline for tramadol ER and placebo ranged from 12-16 and 7-10 points, respectively. Significantly higher proportion of tramadol ER treated patients achieved MCID versus placebo (p<0.05) as early as week 1 except in the stiffness subscale. By week 12, mean change for each subscale and total WOMAC global score for tramadol ER treated patients were significantly greater versus placebo (p<0.01), however, only higher doses (200-300mg) of tramadol ER treated patients achieved MCID versus placebo (p<0.01)On pain subscale, significantly higher proportion of patients treated with tramadol ER 100mg achieved MCID versus placebo at week 1 and 12 (p<0.05). Conclusion: This analysis showed that treatment with tramadol ER in patients with chronic pain extended to improvements in physical function and stiffness as demonstrated by achieving MCID in all WOMAC scores.