The role of mast cells in pulmonary artery occlusion/reperfusion injury was examined. Lung tissue was obtained from dogs after left pulmonary artery occlusion for 48 h (n = 5) or after similar occlusion followed by 4 h of reperfusion (n = 11). By light microscopy and morphometry, the percentage of mast cells increased 2.4-fold (p < 0.05) in nonoccluded right lungs and 2.9-fold (p < 0.05) in occluded left lungs without reperfusion compared with that in control lungs. After reperfusion, the occluded left lung contained 1.8-fold (p < 0.05) as many mast cells as the nonoccluded right lung and 4.2-fold (p < 0.05) more than that in control lungs. Hydroxyurea did not significantly affect the number of mast cells observed in the right and left lungs after ischemia/reperfusion; 39.8% and 54.4% of the mast cells were degranulated in nonoccluded right lung and occluded left lung preparations, respectively, after left pulmonary artery ischemia/reperfusion (each, p < 0.05 compared with control lungs). The release of eicosanoids into the airways during ischemia/reperfusion injury was also examined. Thromboxane B2 and leukotriene B4 were markedly increased (each, p < 0.05 compared with that in control lungs) in bronchial lavage fluids from both nonoccluded and occluded lungs compared with sham-occluded lungs. Thus, mast cell recruitment and degranulation may play a role in lung ischemia/reperfusion injury.