Chylopericardium is a rare clinical entity in which chylous fluid accumulates in the pericardial cavity. We report a case of primary idiopathic chylopericardium associated with multiple, small cervicomediastinal cystic hygromas occurring in an asymptomatic 43-year-old woman with no history of trauma, thoracic surgery, malignancy, infection or tuberculosis. Echocardiography showed a large amount of pericardial effusions and pericardial fluid analysis revealed inappropriately elevated triglyceride. We did not demonstrate communication between the thoracic duct and the pericardial sac by lymphangiography and chest computed tomography. She successfully responded to 30 days of continuous pericardial drainage and 15 days of a medium-chain triglyceride diet after 30 days of total parenteral nutrition. Follow-up echocardiography 6 months after treatment commencement showed a minimal reaccumulation of pericardial fluid without symptom. We conclude that if a patient is asymptomatic and can well tolerate daily life, surgery including pericardiectomy or ligation of the thoracic duct is not necessarily required.