Objectives For rheumatoid arthritis (RA), there is no consensus on how to define and assess flare. Variability in flare definitions impairs understanding of findings across studies and limits ability to pool results. The OMERACT RA Flare Group sought to identify domains to define RA flares from patient and healthcare professional (HCP) perspectives. Methods Flare was described as a worsening of disease activity of sufficient intensity and duration to consider a change in therapy. International patients and HCPs participated in separate and combined rounds of Delphi exercises to rate candidate flare domains previously generated in patient focus groups. Core domains were defined as those with >= 70% ratings of being 'essential' according to the third/final Delphi exercise. Results The final Delphi included 125 RA patients from 10 countries and 108 HCPs from 23 countries who rated 14 domains. Patients had a mean (+/- SD) age of 56 +/- 12 years and disease duration of 18 +/- 12 years. HCPs included physicians from clinical practice/research and industry, allied health providers and researchers with 17 +/- 11 years experience. Core domains comprised: pain (93%), function (89%), swollen joints (84%), tender joints (81%), participation (81%), stiffness (79%), patient global assessment (76%) and self-management (75%). Fatigue (68%), which did not reach group consensus, will receive additional consideration. Conclusions As part of the process to develop a measure for RA flare, patients and HCPs agreed on eight core domains. Next steps include identifying items to assess domains and conducting studies to validate and refine a new measure.