Purpose: Iloprost, a prostacyclin analogue with a prolonged plasma half-life, has beneficial effects on chronic pulmonary hypertension, but its effects on acute lung injury are not well known. We investigated whether iloprost infusion through the pulmonary artery during aortic cross-clamping prevents pulmonary dysfunction after cardiopulmonary bypass (CPB) by measuring inflammatory cytokine levels (IL-1β, IL-6, IL-8, TNF-α), white blood cell (WBC) counts from the left atrium and lung biopsies. Materials and Methods: Twenty elective patients undergoing coronary bypass surgery were divided into two groups. The study group (n=10) received iloprost infusion (2 μg/kg/min) during CPB via a catheter inserted into the pulmonary artery. The control group (n=10) underwent a standard CABG operation. Broncho-alveolar lavage (BAL), respiratory function tests, blood gas measurements, interleukin-1β (IL-1β), interleukin-6 (IL-6), interleukin-8 (IL-8), and tumor necrosis factor alpha (TNF-α) and white blood cell counts were determined. Blood samples were collected from the left atrium through the right superior pulmonary vein before commencing CPB (T1), at the 5th (T2) and 20th minutes (T3) of aortic cross-clamping, and after weaning from CPB (T4). Blood for WBC counts and lung biopsy specimens were collected after weaning from bypass. Results: In the study group FEV1/FVC and pCO2 levels were better than those in the control group. There was a greater inflammatory cellular increase in the BALs of the control group. IL-1β levels at T4 were significantly lower in the study group (0.87 ± 0.16 vs. 1.17 ± 0.24, p Conclusion: Iloprost infusion via a catheter introduced through the pulmonary artery during the aortic cross-clamp period decreases the inflammatory response and acute lung injury after CPB.