We analyzed edema fluid in two cases of reexpansion pulmonary edema during thoracotomy. High value of the fluid to plasma protein concentration ratio indicates an increase in pulmonary microvascular permeability. There were marked increases in polymorphonuclear leukocyte (PMN) count and concentration of PMN-elastase in edema fluid. There were also increases in concentrations of thromboxane B2 and 6-keto-PGF1-alpha in both edema fluid and plasma. These findings strongly suggest that the mechanism of reexpansion pulmonary edema is an inflammatory response and that PMNs in the reexpanded lung may play a role in the increase in permeability.