OMERACT began work over a decade ago on a consensus effort to identify optimal outcome measures for knee and hip osteoarthritis clinical trials. Recent evidence indicates extensive variation in outcome measures used in clinical trials of knee and hip arthroplasty published since 2000. This heterogeneity leads to confusion, not only for conducting systematic reviews but also for applying evidence to clinical practice. Given the extensive psychometric research conducted in the past 2 decades, the timing seems ideal to design and implement a study to develop consensus on optimal outcome measures for hip and knee arthroplasty trials. We describe a Delphi survey design and an approach for synthesizing the extensive psychometric literature on the outcome measures used in hip and knee arthroplasty trials. Plans for dissemination of the findings are also discussed. This proposed study could have an important influence on the design and reporting of future randomized trials of knee arthroplasty.