Twenty-three patients with pulmonary hypertension underwent right cardiac catheterization and measurement of their response to a short-term infusion of prostacyclin. Pulmonary vasodilation with a greater than 20 percent fall in PVR occurred in all five patients with primary pulmonary hypertension where the predominant lesions were plexogenic; in three out of five patients where the predominant lesions were thrombotic; in three out of five patients with pulmonary hypertension and obstruction of proximal vessels; in three out of five patients with COPD; in both patients with pulmonary fibrosis due to sarcoidosis; and in the one patient with pulmonary veno-occlusive disease, where the pulmonary vasodilation was offset by a fall in arterial oxygen content. Prostacyclin is a safe and effective drug to use for testing the response of the pulmonary circulation to a vasodilator in pulmonary hypertension due to different causes. Short-term testing for a vasodilator response, with a view to instituting long-term therapy, should not be restricted to those patients with primary pulmonary hypertension due to plexogenic pulmonary arteriopathy.