Colloid osmotic pressure (COP) was followed postoperatively in 55 randomized patients. After minor operations and short-term infusion therapy only small changes of the COP could be observed and it was concluded that after such operations COP measurement is unnecessary. After major surgical interventions, however, COP measurement gave valuable hints. It was shown that even in the case of moderate blood loss replaced by crystalloids an abnormally low COP did not occur. The same applied also to preoperative hemodilution. It was unnecessary to substitute the withdrawn blood with a colloid solution. In addition, COP measurement helped to avoid expensive albumin administrations, and indicated colloid overload in cases of pulmonary edema.