 It has been demonstrated that 2 year survival for CTEPH patients with mean pulmonary arterial pressuremPAP 50 mm Hg was only 20, however, preliminary data from Sheffield Teaching Hospitals suggest that survival is significantly better than this. Until recently surgery was the only treatment for CTEPH in selected patients. Studies have shown the 10 year survival in patients with CTEPH after PEA surgery was 74. More recently, for patients with inoperable CTEPH, drug therapy has been shown to improve exercise capacity. Despite recent publications the natural history and mechanisms underlying CTEPH are poorly understood. 