A pulmonary embolus and a bilateral thrombosis of the internal carotid artery caused the sudden death of two young adults with a congenital venous abnormality. The study of 49 patients suffering from a Klippel-Trenaunay syndrome showed that 11 out of 49 patients, or 22.5 p. cent had thrombosis problems: namely 7 pulmonary emboli and 8 deep thrombophlebitis. Phlebography did not permit to discover any specific anatomical abnormality. 5 out of 11 patients presented avalvular and dilated deep veins. 6 out of 11 presented angioma of the pelvis. In the group of patients with Klippel-Trenaunay who never presented a deep venous thrombosis, the same signs are noted in one third of the cases. It is possible that coagulation insufficiency may explain the high number of thrombophlebitis. However, an extended analysis of 11 patients did not permit to discover any abnormality. But, subsequently, a study of the fibrinolytic activity demonstrated a normal fibrinogen level, while the fibrinopeptide A level was markedly elevated in each case, with an abnormal thrombin activity.