The aim of this study was to compare the relationship between clinically detected swelling and effusion diagnosed by ultrasonography (US) in elbow joints in patients with rheumatoid arthritis (RA). Fifty consecutive patients with RA entered the study and 20 healthy persons formed a control group. Altogether 100 elbow joints of the RA patients and 40 of the controls were studied. All the clinical assessments were performed by one doctor and the US investigations by the other and they were blinded to each others results. In 77 elbow joints of the RA patients the clinical assessment and the US gave similar results, whereas they differed in the remaining 23 joints. The kappa coefficient between these investigations was 0.371. In the control group no elbow joint showed either swelling in the clinical assessment or effusion in the US investigation. The results of this study indicate that clinical assessment of swelling and evaluation of effusion by US in elbow joints in patients with RA show only fair agreement. Thus, US may improve the accuracy of diagnosis of synovitis in many cases in these patients.