Idiopathic chylopericardium is a rare disease characterized by filling of the pericardial cavity with chylous fluid and has no evident cause. Secondary chylopericardium usually results from injury or damage to the thoracic duct. The most common causes of secondary chylopericardium are trauma, thoracic or cardiac surgery, and congenital lymphangiomatosis. Conservative or surgical treatment can be pursued; however, surgical treatment is required if conservative treatment is unsuccessful. Pericardiocentesis plays a crucial role in the definitive diagnosis of chylopericardium. However, although a serious complication, its occurrence is infrequent. Non-invasive methods, such as computed tomography (CT), could be useful in predicting the color or characteristics of pericardial effusion. A 37-year-old Japanese woman presented to our hospital with a cough that persisted for 1 week. Echocardiography revealed pericardial effusion, which was diagnosed as acute pericarditis and treated with loxoprofen. However, pericardial effusion increased, and the patient presented to the emergency room with cardiac tamponade 1 month later. Pericardiocentesis was performed, which confirmed that the pericardial effusion was chylopericardium. Lymphatic scintigraphy did not show any connection between the thoracic duct and pericardial cavity, and the patient was diagnosed with idiopathic chylopericardium. The patient underwent continuous drainage for 11 days. After completion of cardiac drainage, the patient was discharged from the hospital without any exacerbation. The CT attenuation value of the pericardial fluid was 11.00 Hounsfield units (HU). Compared with the other causes of pericardial effusions encountered at our hospital, the HU on CT scan of pericardial effusion was low in our study and similar to the values on CT scan of chylous ascites reported in previous studies. Although idiopathic chylopericardium is rare, it should be considered an important cause of pericardial effusion. Pericardiocentesis is necessary for definitive diagnosis; however, the CT findings of pericardial effusion may help predict the presence of chylous fluid.