We report a case of idiopathic hypereosinophilic syndrome (HES) presenting with deep vein thrombosis and subsequent massive pulmonary embolism. The eosinophilia and thromboembolism improved dramatically with systemic corticosteroid and anticoagulation therapy. However, the early and rapid tapering of corticosteroid, in a state of normal eosinophil count and therapeutic PT level, aggravated thromboembolism. An immediate retrial of corticosteroid made disease improve again. The etiology of HES and the pathogenesis of thrombus formation in HES still remain uncertain but corticosteroid in treatment of HES is yet cardinal and should be maintained for a prolonged period of time.

Keywords: Hypereosiniphilic syndrome, Thrombosis, Pulmonary embolism