Objective. To assess the reliability and clinically meaningful thresholds of intermittent and constant osteoarthritis pain (ICOAP) score, the Knee injury and Osteoarthritis Outcome Score Physical function Short-form (KOOS-PS), the Hip disability and Osteoarthritis Outcome Score Physical function Short-form (HOOS-PS), and the Quality of life subscales of HOOS/KOOS (HOOS-QOL/KOOS-QOL) in patients with knee or hip arthritis. Methods. One hundred and ninety-five patients (141 knee, 54 hip) seen at 2 orthopedic outpatient clinics with a diagnosis of knee or hip OA completed patient-reported questionnaires (ICOAP pain scale, KOOS-PS, HOOS-PS, KOOS-QOL, HOOS-QOL) at baseline and 2-week followup. Reliability was assessed using intraclass correlation coefficients (ICC). We calculated minimum clinically important difference (MCID) and moderate improvement in the subgroup that reported change in the status of their affected joint. Results. The reliability as assessed by ICC was as follows: ICOAP pain scale, 0.63 (0.48, 0.74) in patients with knee arthritis, and 0.86 (0.73, 0.93) for hip arthritis; KOOS-PS, 0.66 (0.52, 0.77); HOOS-PS, 0.82 (0.66, 0.91); KOOS-QOL, 0.79 (0.69, 0.86); and HOOS-QOL, 0.67 (0.42, 0.83). MCID and moderate improvement estimates in patients with knee arthritis were ICOAP pain scale, 18.5 and 26.7; KOOS-PS, 2.2 and 15.0; and KOOS-QOL, 8.0 and 15.6. A smaller sample in patients with hip arthritis precluded MCID and moderate improvement estimates. Conclusion. We found that ICOAP pain and KOOS-PS/HOOS-PS scales were reasonably reliable in patients with hip OA. Reliability of these scales was much lower in patients with knee arthritis. Thresholds for clinically meaningful change in pain or function on these scales were estimated for patients with knee arthritis. The Journal of Rheumatology Copyright © 2014. All rights reserved.