In two patients with a diffuse abnormality of capillary permeability possibly related to circulating endotoxin, loss of plasma into the tissues (capillary leak syndrome) produced hypovolemic shock, generalized edema, hemoconcentration, and florid pulmonary edema. Pulmonary edema fluid (PEF) was collected and its chemical composition compared to plasma. A number of solutes including the various plasma proteins were in near chemical equilibrium between plasma and PEF. Intravenous administration of dextran 70 (molecular weight 70,000) and dextran (molecular weight 500,000) (in one patient), led to accumulation of these compounds in PEF at a rate consistent with abnormally high pulmonary capillary permeability. These cases document the development of pulmonary edema secondary to increased pulmonary capillary permeability. Possibly, a number of pulmonary diseases (collectively called adult respiratory distress syndrome) result from increased pulmonary capillary permeability, increased alveolar epithelial permeability, or abnormalities of pulmonary interstitial solute removal.