Only five instances of chylopericardium following cardiac surgery have been reported in the literature previously. We encounted this complication in a patient who was operated on for secundum atrial septal defect. The patient readmitted one month after discharge because of large amount of chylous pericardial effusion. Conservative treatment of pericardiostomy drainage and parenteral hyperalimentation was continued for 3 weeks without improvement. Partial pericardiectomy and pericardiopleural window was done with success and no recurrence of chylopericardium was observed upto 3 months after surgery. We think this is the first case report of chylopericardium after open heart surgery in Korea.