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Hypertension of the pulmonary (lung) blood vessels affects large and small arteries and, less often, large and small veins. The high pressure within affected vessels has been associated with constriction of the vessel wall. In the past, primary pulmonary hypertension was classed as being due to thrombosis (clot formation) or vasculitis (blood vessel inflammation). The latter condition was thought to have an autoimmune basis, and to be characterized by the proliferation of fibrous material and the presence of autoantibodies (antibodies inappropriately produced against the body's own molecules). Accordingly, vascular damage might be caused by thickening of arterial walls (by fibrous material), rather than by the constriction of muscle fibers in arterial walls. This classification had no scientific basis, and a recent study described both thrombotic and plexiform (with a network of fibrous tissue) lesions in affected patients. Anticoagulants, which decrease thrombosis, are associated with clinical improvement of this condition. Responses to vasodilators (which relax vessel walls) are generally poor, perhaps, because vessel wall thickening, rather than vasoconstriction, is the problem, or because most patients with primary pulmonary hypertension are seen in advanced stages of disease. Vasodilators often affect systemic vessels (but not pulmonary vessels), and may cause adverse changes in cardiac output and contractility. Survival varies from months to 40 years, reflecting the severity of the condition. The prognosis of pulmonary hypertension is most strongly related to the function of the right ventricle (which pumps blood to the lungs). Echocardiography is useful in assessing heart function in affected patients. In cases of right ventricular failure, heart and lung transplantation provides the only hope of long-term survival with good quality of life. Improvements in transplant management and lung surveillance have improved three-year survival to 65 percent. Patients who are young and have no other disease are most likely to benefit. Further understanding of the physiology of arteries, endothelial cells (which line vessels), and hormones (which regulate them) are needed to develop more successful treatments for primary pulmonary hypertension. (Consumer Summary produced by Reliance Medical Information, Inc.)