Background Prostaglandin analogs have been shown to decrease mean pulmonary arterial pressure (mPAP) and pulmonary vascular resistance (PVR) and to increase cardiac output in patients with lung disease.However, there is a concern that prostanoid analogs may worsen ventilation perfusion (V/Q) mismatch and hypoxia in these patients.The role for inhaled prostanoid analogs in patients with pulmonary hypertension (PH) due to lung diseases remains to be defined.The aim of the study was to investigate the acute effects of inhaled iloprost on V/Q matching, and hemodynamics in patients with pulmonary hypertension (PH) due to chronic lung diseases.