Background/Aims : Pericardial effusion (PE), which is more than a moderate amount, is usually accompanied with various underlying illnesses. However, there have been few reports on the long-term follow-up results of these cases, and even in the studies where the etiologies were well presented. Methods : 64 consecutive patients (mean age: 66.3 years, 23 males) with more than a moderate amount of PE, as confirmed by echocardiography, were analyzed for pericardial fluid and this was diagnosed according to ESC Executive Summary. The mean follow-up duration was 1.95±1.78 years and the final survival was assessed in November, 2006. Results : The etiologies were composed of malignancy-related PE (MRPE: 29.7%), tuberculosis pericarditis (40.6%), idiopathic pericarditis (18.8%), hypothyroidism (7.8%), and miscellaneous (3.1%). Mortality occurred in 21 cases (32.8%), of which 15 cases were attributed to MRPE. In all patients, the mean survival duration was 372±247.9 days after diagnosis, and the mean survival duration was 253±221.5 days after pericardiocentesis. Cardiac tamponade, constrictive pericarditis and recurrent pericarditis were presented in 15 cases (23.4%), 9 cases (14.1%) and 2 cases (3.1%) respectively. The chemistry findings of PE were of no use to differentiate the etiologies. Conclusion : Pericardial effusion of more than a moderate amount often manifested urgent symptoms such as cardiac tamponade and intractable dyspnea, and these conditions require therapeutic pericardiocentesis more frequently than diagnostic tests. The prognosis is usually subordinate to the progression of the underlying illness, and especially in case of MRPE. The occurrence of constrictive pericarditis should be monitored carefully. (Korean J Med 74:154-161, 2008)