Primary pulmonary hypertension (PPH), in which no cause for a rise in pulmonary artery pressure can be found, is characterized by a "restriction" of the peripheral pulmonary vascular bed. Treatment is directed at the consequences of this, namely, the low cardiac output. Oral anticoagulants appear to improve prognosis, as in other low cardiac output states, by preventing intravascular coagulation. Oral vasodilators are not uniformly successful in improving cardiac output. Acute intravenous infusion of prostacyclin (PGI2) lowers both pulmonary and systemic vascular resistance in normal subjects and patients with PPH. Long-term infusion in patients with a poor prognosis, that is, with a PA saturation O2 less than 63%, has, in an uncontrolled study, improved symptoms and rate of maximal oxygen consumption (V max O2) during progressive exercise testing. Its more general use in PPH awaits controlled studies, but there is a possibility that, in addition to its vasodilator properties and the ability to impede platelet aggregation and adherence, PGI2 long term may influence the progressive nature of PPH.