Perioperative hemodilution is one way of minimizing the morbidity and mortality associated with blood transfusion. A review of the literature indicates that, for many patients, substitution of crystalloid solutions for surgical blood loss is safe and physiologically advantageous as long as euvolemia is maintained. Increased stroke volume is the principal mechanism responsible for augmented tissue oxygen delivery. Based on this review the surgical population can be divided into three groups: most patients under 60 years of age (group 1); those with important cardiac disease (group 2); and the elderly (group 3). Patients in group 1 can safely undergo hemodilution to a hematocrit of 0.25; patients in the other groups should probably not be subjected to hemodilution. Implementation of this proposal could diminish the risk and expense of many surgical procedures.