OBJECTIVE We evaluated fibromyalgic RA to determine its clinical impact, identification using core clinical assessments and influence identifying active disease using disease activity scores (DAS-28). METHODS We examined the impact and identification using core clinical assessments (tender minus swollen joint counts) of fibromyalgic RA (> or =11 tender points) in initial (105 patients) and replicate (100 patients) cohorts. Receiver operator characteristic (ROC) curves optimized the cut-off points using tender minus swollen joint counts; their validity was confirmed in a routine practice cohort (321 patients). We evaluated whether fibromyalgic RA affected the identification of active disease using DAS-28 (> or =5.1) and the clinical disease activity index (CDAI). RESULTS A total of 18/105 and 12/100 patients in initial and replicate cohorts, respectively, had fibromyalgic RA. This was identified by > or =7 tender minus swollen joint counts with 83% sensitivity and 80% specificity in the initial cohort (72 and 98% in replicate, respectively) and ROC area under the curve 0.80 (0.94 in replicate). 'Fibromyalgic' RA (tender point scores in initial and tender minus swollen joints in clinical practice cohorts) had higher DAS-28, pain, fatigue and HAQ scores. More fibromyalgic RA patients had active disease by DAS-28 (odds ratio 14.3; 95% CI 5.5, 37.1; and CDAI 17.2; 95% CI 6.1, 48.5); conventional assessments (three or more tender joints; three or more swollen joints; ESR > or =28 mm/h) showed no difference (1.75; 95% CI 0.72, 4.3). CONCLUSIONS Fibromyalgic RA affects 12-17% of RA outpatients and results in worse functional outcomes. DAS-28 scores over-interpret active disease in fibromyalgic RA.