A postoperative increase in the pulmonary extravascular fluid volume (PEFV) and a disturbance of pulmonary gas exchange are regularly observed in patients undergoing cardiac surgery. These alterations, due to a reduction in the colloidal osmotic pressure (COP) increase the morbidity and mortality when crystalloid solutions are infused. This study was based on 90 patients infused with 1,500 cc of crystalloid solution without albumin. The COP, total proteins, albumin and globulin were determined in the plasma preoperatively, during bypass and immediately postoperatively, in order to define a formula allowing calculation of the COP from the plasma proteins to avoid reaching dangerously high figures. The authors concluded that there is a direct correlation between COP and total proteins (total proteins = 1.36 + 0.24 COP) and a potential curve with an adjustment point between COP and albumin (albumin = 0.23 COP0.93). Determination of the COP from globulins is not recommended. The authors elaborated two normograms for everyday clinical use. These formulae allow the calculation of COP in children, elderly subjects and patients in a critical condition, thereby avoiding dangerous levels of COP during infusion of crystalloid solutions.