Hyaluronan (HA), a large glycosaminoglycan found in the extracellular matrix, has major roles in lung and vascular biology and disease. However, its role in idiopathic pulmonary arterial hypertension (IPAH) is unknown. We hypothesized that HA metabolism is abnormal in IPAH. We measured the plasma levels of HA in IPAH and healthy individuals. We also evaluated HA synthesis and the expression of HA synthases and hyaluronidases in pulmonary artery smooth muscle cells (PASMCs) from explanted lungs. Plasma HA levels were markedly elevated in IPAH compared to controls [HA (ng/ml, mean±SD): IPAH 325±80, control 28±9; p=0.02]. In vitro, unstimulated IPAH PASMCs produced high levels of HA compared to control cells [HA in supernatant (ug/ml, mean±SD): IPAH 12±2, controls 6±0.9; p=0.04]. HA levels were also higher in IPAH PASMC lysates. The increased HA was biologically relevant as shown by tissue staining and increased HA-specific binding of mononuclear cells to IPAH compared to control PASMCs [Number of bound cells x10⁴ (mean±SD): IPAH 9.5±3, control 3.0±1, p=0.01]. This binding was abrogated by the addition of hyaluronidase. HA synthase2 and hyaluronidase2 were predominant in control and IPAH PASMCs. Interestingly the expressions of HA synthase2 and hyaluronidase2 were about 2 fold lower in IPAH compared to controls [HA synthase2: IPAH 4.3±0.02, control 7.8±0.1; p=0.0004; hyaluronidase2: IPAH 4.2±0.06, control 7.6±0.07; p=0.008]. Thus, patients with IPAH have higher circulating levels of HA, and PASMCs derived from IPAH lungs produce more HA compared to controls. This is associated with increased tissue levels and increased binding of inflammatory cells suggesting a role for HA in remodeling and inflammation in IPAH.