Chylopericardium is a rare complication of cardiac surgery. It may be caused by a lesion in the thoracic duct or its tributaries or by thrombosis in the confluence of the jugular and left subclavian veins, obstructing the drainage of the thoracic duct. The treatment may be conservative or surgical, depending on the duration and on the volume of the effusion. We report the case of a 24-year-old female, who, in the late postoperative period of mitral valve replacement (bioprosthesis), was hospitalized with cardiac tamponade due to the presence of chylopericardium. The clinical findings and treatment performed are discussed.