It has been generally accepted that an adequate oxygen-carrying capacity can be achieved with a hemoglobin concentration of 7 g/dl, as far as the patient's intravascular volume is sufficient to allow tissue perfusion. To guarantee patient's safety in the operating theater, patient's oxygenation, ventilation, circulation and temperature, which enable oxygen utilization in tissues, should be monitored vigilantly and ensured strictly. This is also true when taking care of anemic patients outside the operating theater, because failure of these functions in anemic patients leads directly to tissue hypoxia. Besides the standard monitoring, measuring oxygen carrying capacity/consumption parameters and gastric/sigmoidal intramucosal pH have been shown to be helpful to estimate tissue oxygenation. Therefore, safe levels of hemoglobin concentration should be determined according to the ability of doctors and nursing staffs to evaluate and to maintain patient's systemic and tissue oxygenation as well as to the patient's pathophysiological conditions.