Pulmonary hypertension may occur as a primary disorder of the pulmonary vasculature or secondary to a variety of cardiac or pulmonary diseases. The reversibility of pulmonary hypertension is dependent on the relative contribution of reversible vasoconstriction and irreversible structural changes in the pulmonary vessels. Despite recent advances in the understanding of pulmonary vascular physiology, knowledge of the pathogenesis and natural history of pulmonary hypertension has been limited by an inability to measure pulmonary arterial pressure noninvasively. Thus, when patients have symptoms or signs of pulmonary hypertension, the disease is usually at an advanced stage. It is possible that early in the course of hypoxic pulmonary disease, pulmonary hypertension may be protective in optimizing matching of ventilation and perfusion. It is not known at what point pulmonary hypertension per se becomes harmful. Certainly, treatment directed at underlying cardiac or pulmonary disease is indicated. It also seems reasonable to treat severe degrees of pulmonary hypertension complicated by right ventricular dysfunction. With the advent of orally effective pulmonary vasodilators, direct treatment of primary and secondary pulmonary hypertension may now be possible. Hopefully, with careful clinical evaluation of the response to vasodilator therapy, we will learn whether these drugs prolong life and reduce morbidity in primary and secondary pulmonary hypertension. In the meantime, much more information is needed regarding the mechanisms of acute pulmonary vasoconstriction and sustained pulmonary hypertension. In addition, a means of early identification of patients with mild hypertension is needed.(ABSTRACT TRUNCATED AT 250 WORDS)