Ten patients with severe primary pulmonary hypertension and pronounced disability who were unresponsive to oral vasodilators were treated with intravenous epoprostenol (prostacyclin). All had been referred for heart and lung transplantation. Short term administration of epoprostenol (mean dose 5.5 ng/kg/min) increased the mean cardiac index from 1.8 to 2.2 1/min/m2, improved pulmonary artery oxygen saturation from 48% to 57%, and increased calculated tissue oxygen delivery from 10 to 11.8 ml/kg/min. The mean pulmonary vascular resistance fell by 18% while mean systemic artery pressure fell by 32%. Pulmonary artery pressure rose in only two patients. Continued intravenous infusion of epoprostenol for 1-25 months was associated with subjective and clinical improvement. Exercise tolerance improved as measured by an increase in the maximum rate of oxygen consumption during progressive exercise testing. In those six patients who were able to exercise before treatment it rose from a mean of 7 to 15 ml/kg/min. Those who had been unable to exercise before treatment achieved comparable rates of oxygen consumption after treatment. Two patients died on treatment, three have undergone heart-lung transplantation, and in five the treatment is continuing. Complications included episodes of septicaemia and ascites. In this uncontrolled study of patients with severe pulmonary hypertension epoprostenol seemed to offer a means of optimally dosing the patients with a vasodilator to reduce pulmonary vascular resistance and thus increasing cardiac output and oxygen tissue delivery. There was no evidence to suggest that this treatment influenced the progress of the disease.