A case of reexpansion pulmonary edema during thoracotomy is presented. A high edema fluid-plasma protein concentration ratio suggested an increase in pulmonary vascular permeability. A marked increase in the percentages of polymorphonuclear leukocytes (PMN) and high concentration of PMN-elastase in edema fluid suggested activated PMN-mediated vascular endothelial injury, which caused an increase in permeability. Edema fluid and plasma contained high concentrations of Thromboxane B2 and 6-keto-PGF1 alpha at the stage of increased permeability. This suggested these chemical mediators participated in the mechanisms of increased permeability in reexpansion pulmonary edema.