The relationship between the serum colloid osmotic pressure (COPs), pulmonary artery wedge pressure (PWP), and pulmonary edema fluid colloid osmotic pressure was studied in six critically ill patients with fulminant noncardiogenic pulmonary edema. The relationship between COPs and PWP was also studied in 36 critically ill patients without pulmonary edema. The COPs-PWP gradient was normal in those patients without pulmonary edema. Three patients with noncardiogenic pulmonary edema had markedly reduced COPs-PWP gradients secondary to decreases in COPs. Their pulmonary edema fluid colloid osmotic pressure averaged 61% that of their serum colloid osmotic pressure. Three patients with noncardiogenic pulmonary edema had normal COPs-PWP gradients. Their pulmonary edema fluid colloid osmotic pressure averaged 92% that of their COPs. Noncardiogenic pulmonary edema in the critically ill patient may be caused by either a decrease of COPs-PWP gradient or an increase in capillary membrane permeability.