Klippel - Trenaunay - Weber syndrome (KTWS) is a congenital condition characterized by a triad of capillary malformations of the skin, soft tissue and bone hypertrophy resulting in limb enlargement, and abnormalities of arteriovenous and lymphatic systems of the affected limb. In this case, we present a patient with KTWS receiving chronic anticoagulation that had a massive pulmonary embolism and was successfully treated with thrombolytic therapy. The purpose of this case is to educate readers about this uncommon condition and to increase awareness, recognition and timely treatment of its most common complications, namely thrombosis and pulmonary embolism.