OBJECTIVE To determine the efficacy of continuous intravenous infusion of prostacyclin (epoprostenol) in primary pulmonary hypertension. METHODS Randomized trial with 8-week treatment periods and nonrandomized treatment for up to 18 months. METHODS Four referral centers. METHODS Sequential sample of 24 patients with primary pulmonary hypertension. Nineteen patients completed the study. Four patients died and one left the study because of adverse effects (pulmonary edema). METHODS Continuous intravenous prostacyclin administered by portable infusion pump at doses determined by acute responses during baseline catheterization in ten patients. Nine patients were treated with anticoagulants, oral vasodilators, and diuretics. RESULTS Starting with a baseline value for total pulmonary resistance of 21.6 units, there was a decrease of 7.9 units (95% CI, -13.1 to -2.2; P = 0.022) in the prostacyclin-treated group after 8 weeks; there was virtually no change in the conventional therapy group (from 20.6 to 20.4 units, not significant). Six of ten prostacyclin-treated patients who completed the 8-week study period had reductions in mean pulmonary artery pressure of greater than 10 mm Hg, whereas only one of nine in the conventional treatment group had a similar response (P = 0.057). Nine patients receiving prostacyclin for up to 18 months have persistent hemodynamic effects, although dose requirements have increased with time. Complications have been attributable to the drug delivery system. CONCLUSIONS Prostacyclin produces substantial and sustained hemodynamic and symptomatic responses in severe primary pulmonary hypertension and may be useful in the management of some patients with this disease.