Chylopericardium is a rare disorder that may be primary (idiopathic) or secondary to injury of the thoracic duct or thymus gland. Pediatric cardiac operations are more commonly related to this complication because thymus gland is very active in this population and atrophies in the adult patients. We present a case of chylopericardium after mitral valve repair for rheumatic disease, due to thymus gland tributaries injury.