Clinical applications of nuclear medicine in pulmonary hypertension (PH) include: 1) Differentiation of primary from chronic thromboembolic form of PH by mismatched ventilation/perfusion defects, 2) Quantification of right ventricular ejection fraction to estimate cardiac functional derangement, 3) Estimation of right ventricular hypertrophy or pressure overload by increased accumulation of radioactive myocardial perfusion agents, and 4) Estimation of myocardial sympathetic nervous function or fatty acid metabolism. Gravity dependent shift of pulmonary perfusion detected by perfusion scintigraphy has also been utilized for estimating severity of PH. Positron emission tomography (PET) has visualized distribution of pulmonary adrenergic receptors and angiotensin converting enzyme. PET, in addition to F-18 FDG lung tumor PET, should be watched for because it may visualize and quantify other bioactive substances and receptors in the lung.