The Flare instrument (FI) is a French self-administrated questionnaire used to identify flares in disease activity in patients with rheumatoid arthritis. In addition to a total score, the FI has two subscales: one relating to joint symptoms and one relating to general symptoms. The objective of this study was to translate and adapt the French FI into Danish and to determine the reliability of the FI in a consecutive cohort of patients with RA. The FI was translated according to international guidelines, tested among 10 patients and 5 health professionals, and adapted. Test-retest reliability was determined by the standard error of the measurement (SEM) and the intra class correlation coefficients (ICC). The FI was administered to 50 patients with rheumatoid arthritis from an outpatient clinic of a university hospital and re-administered after 10 days. The patients had a mean age of 65.3 years (SD 12.0) and mean disease duration of 18.1 years (range 2-47 years). We found an excellent reliability with ICC higher than 0.95 and SEM between 0.44 and 0.63. Best reliability was found in the total FI score. Thus, the results of the present study show that the FI is a feasible and reliable tool for evaluation of flares in patients with rheumatoid arthritis.