As is often the case in medicine, the risk to benefit ratio of blood transfusion requires careful examination. The decision to transfuse a patient cannot be based only on the haemoglobin or the haematocrit concentration. In a given clinical situation, tolerance to anaemia will depend on the ability of the patient to recruit his compensatory mechanisms in accordance with his metabolic demand. Maintenance of circulating volume is of paramount importance. In the absence of a universal transfusion trigger, decision to transfuse a patient with an haemoglobin concentration between 7.0 and 10.0 g dL−1 depends primarily on clinical judgement. With the exception of emergency situations, blood transfusion will be realized on a unit-by-unit basis, and the efficacy of treatment should be evaluated after each transfused unit.