The relationship between colloid osmotic pressure (COP) and the severity of cardiopulmonary failure was investigated in 99 consecutive patients admitted to our Shock Unit including 41 survivors and 58 fatal cases. The COP was significantly lower (p less than 0.001) in fatal cases in comparison to survivors. None of 21 patients in whom COP was less than 10.5 torr survived. A progressive increase in survival with typical S-shaped distribution was observed as COP increased from 10 to 19 torr, above which all patients survived. These preliminary observations demonstrated that reduction in COP in critically ill patients is associated with increases in mortality. The mechanisms by which lowering of COP may be related to fatal progression of cardiopulmonary failure is the subject of continuing study.