Pulmonary arterial hypertension (PAH) is a progressive disease, with a poor prognosis. The pathophysiologic mechanism of PAH is unknown, but may involve both tissue remodeling and inflammatory processes. Hyaluronan (HA) is a large glycosaminoglycan polymer and a major component of the extracellular matrix. In the present study, we measured plasma HA levels in PAH associated with systolic congestive heart failure (CHF, n = 16) or chronic obstructive pulmonary disease (COPD, n = 18). The control group was consisted of 14 healthy individuals without pulmonary or cardiovascular disease. Plasma HA levels (ng/mL) were determined in all patients by an enzyme linked HA binding assay. Pulmonary arterial pressure (PAP) was calculated in echocardiography (mmHg). Pulmonary arterial pressures were significantly higher in CHF and COPD (CHF: 55.0 ± 11 mmHg and COPD: 62.5 ± 21 mmHg, p < 0.001 for each), compared to the control group (25.4 ± 5.9 mmHg). Plasma HA levels were significantly higher in CHF (73.0 ± 37.5 ng/ml, p = 0.007) and COPD (87.3 ± 53.2 ng/ml, p = 0.001) compared to control patients (26.2 ± 8.4 ng/ml). There was no significant difference in plasma HA levels between the CFH and COPD groups (p = 0.690). In COPD, plasma HA levels were significantly correlated with PAP, left atrium diameter. There was no significant correlation between plasma HA levels and age or with echocardiography parameters in CHF. Both CHF and COPD are associated with increased plasma HA levels. Elevated plasma HA may contribute to the development of PAH.