“Flare” is a term that is commonly used by both patients and clinicians to describe a debilitating worsening of symptoms. While conceptually recognized, there is limited formative research to identify the measures of clinical characteristics or other variables that define this state. As treatment strategies and medications become available to achieve low levels of disease activity and remission, measures to guide tapering of medications, or re-treatment when disease appears to worsen, are needed to facilitate clinical trials and to move these treatment approaches from clinical trials to clinical practice. While several different definitions of flare or disease worsening have been used in clinical studies, the validity of these to guide changes in treatment has not been tested. Moreover, the experience of RA patients has not been adequately evaluated to assure understanding of features of disease worsening that are most important, their magnitude, and/or their temporal occurrence and duration, which then cause a patient to seek change in treatment. An Outcome Measures in Rheumatology Clinical Trials (OMERACT) initiative, which includes researchers, clinicians, and patients, is now underway to evaluate these aspects of disease worsening in RA. Timely, data-driven completion of this project with an agreed definition of RA flare is intended to facilitate clinical trials, observational clinical research, and ultimately enhance clinical care.Int J Adv Rheumatol 2009;7(3):85–91.