Two cases of isolated chylopericardium, one idiopathic and the other secondary to the lymphangioma of the pericardium are reported here. In both cases, correct diagnosis was made by radionuclide imaging techniques. When imaging the cardiac pool using 99mTc-in vivo, labeled RBC, a large separation between the heart and surrounding organs was demonstrated; this suggested a presence of massive pericardial effusion. On pericardial imaging taken 24 hours after oral administration of 131I-triolein, a donut-like shape was clearly produced, situated in a gap between the separation, suggesting chylous effusion within the pericardial space. On operating, isolated chylopericardium was confirmed in both cases. To our knowledge, this is the first report of preoperative noninvasive diagnosis of chylopericardium in Japan.