-The mechanisms of right ventricular (RV) failure in pulmonary arterial hypertension (PAH) are poorly understood. Abnormalities in fatty acid (FA) metabolism have been described in experimental models of PAH, but systemic and myocardial FA metabolism have not been studied in human PAH.Methods and Results-We used human blood and RV tissue and non-invasive imaging to characterize multiple steps in the FA metabolic pathway in PAH subjects and controls. Circulating FFAs and long-chain acylcarnitines were elevated in PAH patients versus controls. Human RV long chain FAs were increased and long chain acylcarnitines were markedly reduced in PAH versus controls. Using proton magnetic resonance spectroscopy,in vivomyocardial triglyceride content was elevated in human PAH versus controls (1.4 ± 1.3 %TG vs. 0.22 ± 0.11 %TG, p = 0.02). Ceramide, a mediator of lipotoxicity, was increased in PAH RVs versus controls. Using an animal model of heritable PAH we demonstrated reduced fatty acid oxidation via failure of palmitoylcarnitine to stimulate oxygen consumption in the PAH RV.Conclusions-Abnormalities in fatty acid metabolism can be detected in the blood and myocardium in human PAH and are associated within vivocardiac steatosis and lipotoxicity. Murine data suggests that lipotoxicity may arise from reduction in fatty acid oxidation.