Slow-flow vascular malformations are divided into venous, lymphatic, and combined malformations such as Klippel-Trénaunay syndrome (KTS), which is characterized by venous, lymphatic, and capillary malformations; skeletal hypertrophy; and increased soft tissue in 1 or more limbs. Associated coagulation disorders may produce hemorrhage and thrombosis. Significant complications of KTS include thrombophlebitis and thromboembolic events, which may cause chronic thromboembolic pulmonary hypertension (CTEPH). In this case-control study, Rodríguez-Mañero et al demonstrate that pulmonary arterial hypertension in patients with extensive slow-flow vascular malformations is relatively frequent, and that levels of D-dimer correlate with pulmonary artery systolic pressure. A serious process, CTEPH leads to right ventricular insufficiency and can cause death. An echocardiogram should be performed on all patients diagnosed as having an extensive slow-flow vascular malformation.