To study responsiveness and establish the minimal clinically important differences (MCID) and minimal detectable change (MDC) in patients undergoing total hip replacement (THR) using the Short Form 36 (SF-36) and the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC).We conducted a prospective observational study in three public hospitals of all consecutive patients with a diagnosis of hip osteoarthritis (OA) on waiting lists to undergo THR. Patients completed the SF-36 and the WOMAC (subscales transformed to 0 to 100), which measured the health-related quality of life (HRQoL), before intervention and 6 months and 2 years later, and additional transitional questions, which measured the changes in the joint 6 months postoperatively.Improvements at 6 months after a THR were between 37 (stiffness) and 39 points (pain), depending on the WOMAC domain. The SF-36 domains also showed improvements: physical function (31.91), physical role (33.71), and bodily pain (29.77). From 6 months to 2 years, improvements ranged from 2 to 5 points, except for role physical (13.25). A ceiling effect was detected on some WOMAC domains as well as a floor effect on the SF-36. The MCID ranged from 25.91 (stiffness) to 29.26 (pain) on the WOMAC and from 10.78 (physical role) to 20.40 (physical function) on the SF-36. The MDC ranged from 21.38 (pain) to 27.98 (stiffness) on the WOMAC and from 18.99 (physical function) to 42.05 (social function) on the SF-36.These values indicate expected gains after THR. However, the MCID and MDC values must be viewed cautiously due to the uncertainty of these estimators and should not be considered as absolute thresholds.