Klippel-Trenaunay syndrome (KTS) is a congenital disorder characterized by cutaneous capillary malformations, soft tissue and bone hypertrophy, and multiple capillary, venous or lymphatic malformations. KTS is associated with recurrent thromboembolic events. We reported herein five cases of chronic thromboembolic pulmonary hypertension (CTEPH) associated with KTS (age minimum–maximum 26–50 years old, 3 males/2 females). Hemodynamics showed severe pulmonary hypertension (PH) with pulmonary vascular resistance ranging from 5.6 to 18.3 Wood units (WU), associated with marked clinical impairment (NYHA functional class III or IV in 4 patients). Computed tomography (CT) of the chest and pulmonary angiography confirmed proximal CTEPH accessible to surgical intervention in one patient and distal forms of CTEPH in 4 patients. Evolution after pulmonary endarterectomy showed hemodynamic normalization, while the patients with distal CTEPH had severe outcomes with 2 early deaths after PH diagnosis (44 and 35 months respectively). One patient with distal CTEPH was still alive 16 years after diagnosis on specific PH therapy and one was transplanted after 15 years because of right heart failure (death after 12 months). Histological analysis of the lung explants showed typical chronic thromboembolic material specific for CTEPH. In conclusion, KTS may be complicated by severe CTEPH requiring careful anticoagulation and multidisciplinary follow-up in expert centers to screen for disease potentially accessible to endarterectomy. In the modern management era of CTEPH, balloon pulmonary angioplasty will certainly be an interesting option in patients with inoperable disease.