In a patient with classical rheumatoid arthritis and pericardial involvement, accelerated restriction of cardiac filling resulting from pericardial constriction developed. Pericardiectomy was necessary to relieve this condition. Examination of the synovial and pericardial fluid showed noteworthy decreases in total hemolytic complement (CH50) and C3 levels, while these were normal in the serum. Immunofluorescence staining of the pericardium showed plasma cell infiltration and immune deposit staining of pericardial vessels with IgG, IgM, IgA, or C3. These findings suggest that immune complexes deposition plays an important role in the pathogenesis of this condition.