The rationale for the acute vasodilator test in idiopathic pulmonary arterial hypertension (IPAH) is to identify patients who have a vasoreactive component that justifies the use of non-selective vasodilators. We tested the ability of two different response criteria to identify such patients studying 34 patients with IPAH. The hemodynamic data from the right heart catheterization were collected at baseline and post-administration of inhaled nitric oxide. We describe the results obtained by two different response criteria: (A) a 20% decrease in indexed pulmonary vascular resistance (PVRi) and in mean pulmonary artery pressure(PAPm)(classic criterion); (B) a fall in PAPm of at least 10 mm Hg to a pressure level of 40 mm Hg or lower (revised criterion). Patients who responded according to the revised criterion showed a higher cardiac index (CI) (3.45+/-1.94 vs. 1.99+/-0.44 L/min/m2; p=0.05) and lower PVRi (1247+/-746 vs. 2437+/-1105 dyn cm-5 s m2; p=0.02) compared to non-responders. Responders according to the classic criterion had a statistically significant lower CI (1.59+/-0.40 vs. 2.43+/-1.14 L/min/m2; p=0.03) and higher PVRI (3130+/-1173 vs. 1958+/-980 dyn cm-5 s m2; p=0.04) also compared to non-responders. We conclude that the revised criterion seems to reflect the degree of vascular remodeling more accurately, identifying patients with better preserved cardiac function, possibly in an earlier phase of the disease.