Background: Idiopathic pulmonary arterial hypertension (iPAH) is a rare disease of unknown aetiology characterized by a poor prognosis. Impairment of nitric oxide (NO) synthesis or NO-induced vasorelaxation has been suspected to play a role in the development of iPAH. This study was performed to investigate possible correlations between the plasma levels of the NO-related aminoacids L-arginine, L-citrulline and N-hydroxy-L-arginine (L-NHA) and the severity of iPAH. Methods: In twelve iPAH patients hemodynamics were measured by right heart catheterization, and plasma levels of L-arginine, L-citrulline and L-NHA were determined in blood samples from the pulmonary artery, peripheral artery and peripheral vein by high-performance liquid chromatography analysis. In eight of twelve patients a six minute walk test was performed. Results: Plasma levels of L-arginine strongly correlated to right atrial pressure, cardiac output, cardiac index, mixed-venous oxygen saturation, six minute walk data and NYHA functional class at all sites of blood sampling (p < 0.05). Conclusions: The results suggest a possible role of the NO precursor L-arginine in the pathogenesis of iPAH.