Concurrent analyses of the colloid osmotic pressure of pulmonary edema fluid and serum were performed in two patients with reexpansion pulmonary edema. In addition, pulmonary artery wedge pressure was measured during pulmonary edema in one patient. The colloid osmotic pressure of the pulmonary edema fluid was 73 and 81 percent of the serum value. The pulmonary arterial wedge pressure in one patient was within normal limits (8 mm Hg). Reexpansion pulmonary edema appears to be due to increased pulmonary capillary permeability rather than to hemodynamic mechanisms.