In order to determine the incidence of pericardial effusion in rheumatoid arthritis (RA), echocardiographic studies were performed on patients with classical or definite RA as determined by ARA criteria, none of whom showed any clinical cardiac symptoms at the time of examination. Of 50 RA patients studied so far, fifteen (30%) showed evidence of posterior pericardial effusion, as recorded on the strip charts. Additionally, in one full-blown case of rheumatoid vasculitis, massive accumulation of pericardial fluid was demonstrable. Compared to those without pericardial effusion, patients shown to have pericardial fluid tended to have decreased RBC, Hct, Hgb, serum protein concentration and increased ESR. The statistical difference between the two groups was significant (P<0.005). The most conspicuous difference between the two, however, was noted in the degree of hypoalbuminemia that is more profound in the effusion positive group (P<0.005). The possible cause of fluid retention in the pericardial cavity in RA was briefly discussed.