Pulmonary arterial hypertension (PAH) is a syndrome characterized by lung vascular intimal lesions, smooth muscle layer hypertrophy, perivascular inflammation, and concomitant right ventricular (RV) remodeling which can ultimately lead to the RV function decline known as RV failure (RVF). A key determining factor for RVF development is the RV metabolic state defined by the level of ischemia and glycolysis which constitute a vicious cycle of hypoxia, metabolic shift from glucose oxidation (GO) to glycolysis, increased RV systolic pressure (RVSP), decreased RV output, and further myocardial ischemia. In this context, 2-deoxy-2-[(18)F]fluoro-D-glucose (FDG) positron emission tomography (PET) has been used for the measurement of glucose uptake (GU) as an indicator of glucose metabolism in the right heart and pulmonary vasculature. The focus of this review is the application of FDG PET modality for assessing PAH severity and clinical outcome.