The relationship of serum colloid osmotic pressure (COP) and pulmonary artery wedge pressure (PAWP) to pulmonary edema and mortality was investigated in 76 critically ill patients. Forty patients suffered from circulatory shock and 36 did not. The COP-PAWP gradient and radiographic evidence of pulmonary edema were measured at the time of diagnosis. The COP-PAWP gradient was markedly decreased in both shock and non-shock patients with pulmonary edema. Decrease of the COP-PAWP gradient correlated with mortality only in those patients with shock. The shock patients who died had a significantly higher incidence of pulmonary edema than those who survived, whereas there was no difference in the incidence of pulmonary edema for surviving or dying non-shock patients. We conclude that marked decreases of the COP-PAWP gradient predict pulmonary edema in the critically ill, but predict mortality only for patients with circulatory shock.