Acute vasodilator tests with prostacyclin (PGI2) or inhaled nitric oxide (iNO) are used to select patients with pulmonary arterial hypertension (PAH) who should be treated with oral vasodilators. The haemodynamic effects of PGI2 and iNO are different, and the limits for considering a vasodilator response as significant are controversial. The study was aimed to investigate the diagnostic performance of acute vasodilator testing with iNO and PGI2 in predicting the clinical outcome after 1 year treatment with oral vasodilators. Twenty-seven patients with severe PAH were studied. Nineteen patients were treated with oral vasodilators and their outcome after 1 year was qualified as favourable or unfavourable. The diagnostic performance of vasodilator tests in predicting this outcome was evaluated using receiver operating characteristics (ROC) curves. The acute effects of iNO and PGI2 on pulmonary artery pressure (PAP) were similar. By contrast, PGI2 produced more marked changes on cardiac output and pulmonary vascular resistance than iNO (P<0.05). The evolution at 1 year was favourable in 11 patients and unfavourable in 8. Patients with favourable evolution showed greater decrease of PAP with iNO than with PGI2 (P<0.05). The decrease of PAP with iNO had the greatest predictive value on the clinical outcome (area under ROC curve, 0.83). We conclude that in patients with PAH, acute vasodilator testing with iNO is preferable to PGI2 because it reflects more consistently the changes in pulmonary vascular tone. The acute decrease of PAP with iNO is the best predictor of the long-term response to oral vasodilator treatment.