Rationale: Insulin resistance has emerged as a potential mechanism related to the pathogenesis of idiopathic pulmonary arterial hypertension (IPAH). However, direct measurements of insulin and glucose metabolism have not been performed in patients with IPAH to date. Objectives: To perform comprehensive metabolic phenotyping of humans with IPAH. Methods: We assessed plasma insulin and glucose, using an oral glucose tolerance test and estimated insulin resistance, and b-cell function in 14 patients with IPAH and 14 control subjects matched for age, sex, blood pressure, and body mass index. Body composition (dual-energy X-ray absorptiometry), inflammation (CXC chemokine ligand 10, endothelin-1), physical fitness (6-min walk test), and energy expenditure (indirect calorimetry) were also assessed. Measurements and Main Results: Patients with IPAH had a higher rate of impaired glucose tolerance (57 vs. 14%; P,0.05) and reduced glucose-stimulated insulin secretion compared with matched control subjects (IPAH: 1.3160.76 mU/ml$mg/dl vs. control subjects: 2.2161.27 mU/ml$mg/dl; P,0.05). Pancreatic b-cell functionwas associatedwith circulating endothelin-1 (r = -0.71, P,0.01) and CXC chemokine ligand 10 (r = -0.56, P,0.05). Resting energy expenditure was elevated in IPAH (IPAH: 32 63.4 vs. control subjects: 28.8 62.9 kcal/d/kg fat-free mass; P ,0.05) and correlated with the plasma glucose response (r = 0.51, P ,0.01). Greater insulin resistance was associated with reduced 6-minute walk distance (r = 0.55, P ,0.05). Conclusions: Independent of age, sex, blood pressure, and body mass index, patients with IPAH have glucose intolerance, decreased insulin secretion in response to glucose, and elevated resting energy expenditure. These abnormalities are associated with circulating markers of inflammation and vascular dysfunction.