The present study was undertaken to determine the prevalence of asymptomatic pericardial effusion in a population with end-stage renal failure just prior to the initiation of chronic dialysis, and to determine the effect of long-term dialysis on these effusions. We prospectively studied 50 uremic patients with M-mode echocardiograms prior to initiation of chronic dialysis and followed 33 of these patients after 10 months of intense dialytic therapy. Predialysis effusion was present in 18/50 (36%) patients. Only 3/50 patients had clinical evidence of pericarditis (none of these individuals had an effusion). The incidence of clinical congestion and radiological evidence of volume overload was significantly higher in the patients with an asymptomatic pericardial effusion. Of these, the effusion disappeared on improved in 6 (43%), remained unchanged in 6 (43%), and worsened in 2 (14%). No patients developed new pericardise effusions during chronic dialysis. Changes in effusion size were related to changes in body weight between dialysis treatments (r = 0.39; p less than 0.05). Our data show that asymptomatic pericardial effusions are frequent in uremic patients prior to initiation of dialysis, the etiology of asymptomatic pericardial effusions in these patients appears to be related to volume overload, only 43% of the patients improved their effusions with chronic dialysis.