Increasing evidence on the limited usefulness and the adverse consequences of erythrocyte transfusion has led to a large drop in the number of blood transfusions over the last 20 years. The results of randomised studies suggest that in most haemodynamically stable patients with acute anaemia an Hb transfusion threshold of 4.4 mmol/l for blood transfusion has the same outcomes as a higher transfusion threshold. The effect of blood transfusion in patients with anaemia is not only dependent on their Hb level, but also on other clinical factors that play a role in the balance between oxygen supply and its consumption. The Dutch '4-5-6' rule for indication for blood transfusion takes a number of important clinical factors into account, however, results of recent research suggest that the strict application of this rule will lead to unnecessary transfusions. New research in this area is focused on the quantification of the effect of blood transfusion in various combinations of relevant patient characteristics.