Clinical data from 41 patients with pericarditis or pericardial effusion in acute and chronic renal failure were analyzed in respect to the diagnostic relevance of humoral immune reactions. In patients with pericardial effusion in acute renal failure due to surgery or trauma (n = 10), antimyocardial antibodies were rarely detected. In contrast, in the sera of all patients with pericarditis or pericardial effusion following renal failure in chronic or acute glomerulonephritis (n = 6), complement-fixing antisarcolemmal antibodies of the IgM and IgG classes were observed. In patients in whom pericarditis or pericardial effusion evolved during chronic hemodialysis (n = 25), the incidence of complement-fixing antimyolemmal antibodies was 64%. Only sera with complement-fixing antimyolemmal antibodies induced cytolysis of vital adult cardiocytes in vitro suggesting that the antimyolemmal antibodies may not only play a diagnostic but also a pathogenetic role in ‘uremic’ pericarditis in v