BACKGROUND: The most often used functional classification for categorizing the degree of cardiac disability in patients with chronic left ventricular failure is the NYHAN/WHO system. In Idiopathic Pulmonary Arterial Hypertension [I-PAH], this system although used, has not been studied in detail regarding pulmonary hemodynamic parameters association and for long-term prognosis in each of the NYHA/WHO classes. METHODS: We retrospectively, studied the NYHA/ WHO system in 83 I-PAH patients. Patients were separated according to the response in the acute vasodilator trial in responders [n = 30] and nonresponders [n = 53]. RESULTS: Classes I - II did not represent the minority population for I-PAH patients [58/83 = 60%]. Only mean right atrial pressure [mRAP] and mean pulmonary artery pressure [mPAP] were different among the NYHA/WHO functional classes [p < 0.000 and p <0.012; respectively]. I-PAH patients class I have the probability to be a responder 12.6 times more [CI 95.%: 4.59-40.62; p < 0.000]. The long-term mortality for class I patients was 0.%, for class II: 2.%, for class III: 28.% and for class IV: 63.% [p < 0.0001]. The follow-up change for one grade class of the NYHA/WHO classes at four years was noticed only in 20.% of the I-PAH patients. CONCLUSIONS: NYHA/WHO classes I-II did not represent the minority of I-PAH patients population as has been previously considered. Only mRAP and mPAP were different among the NYHA/WHO classes. The NYHA/ WHO system on the basis of mRAP and mPAP allows to separate classes I-II from III-IV. I-PAH patients class I have 12.6 times more the probability to be a responder and better long-term survival; irrespective of the treatment the prognosis seems to be excellent for this functional class group patients.