To assess the relationship between pulmonary vascular obstruction and hemodynamic status in acute pulmonary embolism (APE) and in chronic thromboembolic pulmonary hypertension (CTEPH), perfusion lung scan and hemodynamic measurements were obtained in 31 consecutive patients with APE and in 45 with CTEPH. Lung scans were scored independently by two experience observers who determined the percentage of vascular obstruction (PVOs). Mean pulmonary artery pressure (PAP) and total pulmonary resistance (TPR) were obtained during right heart catheterization. In patients with APE, measurements were recorded within a 1-hr interval before and 12 hours after thrombolysis. This yielded 62 paired PVOs values with concomitant PAP and TPR measurements. In patients with CTEPH, data were recorded within a 3-day interval. Mean PVOs (%) values were similar in APE and CTEPH patients (59 +/- 13 vs. 58 +/- 15), whereas PAP and TPR were significantly higher in CTEPH patients (51 +/- 17 mmHg and 23 +/- 11 U/m2, respectively) than in APE patients (23 +/- 8 mmHg and 9 +/- 5 U/m2, respectively, p < 0.001). In APE patients, significant hyperbolic correlations were found linking PVOs with PAP and TPR (r = 0.75, p < 0.01 for PAP; r = 0.71, p < 0.01 for TPR). In CTEPH, there were no significant correlations between PVOs and PAP or TPR. For the same level of PVOs, patients with CTEPH had higher PAP and TPR value than patients with APE. In APE without prior cardiopulmonary disease, increases in PAP and TPR are correlated in a nonlinear fashion with the degree of pulmonary vascular obstruction as assessed by lung scanning. In CTEPH patients, the higher PAP and TPR values as compared to APE patients with comparable degrees of PVOs are consistent with previous reports that pulmonary hypertension in CTEPH is due not only to the obstruction of proximal pulmonary arteries but also to remodeling of small distal arteries in nonoccluded areas.