Here we report a case of a primary idiopathic chylopericardium in a 13 years old child. Pericardial effusion was diagnosed because the child suffered chest pain and fatigue. Pericardial drainage was performed and 800mL of chylous fluid was evacuated. Extensive investigations were performed but no cause could be found. Thoracic CT scan, lymphoscintigraphy and MRI did not evidence any communication between the thoracic duct and pericardium. After 2 recurrences of pericardial effusion while the child was on a medium chain triglycerides regimen, it was decided to ligate the thoracic duct and to do a partial pericardectomy. The result was excellent with complete resolution of the pericardial effusion and no recurrence since 3 years.