Allogenic blood transfusion may be required for the treatment of anemia due to a hematologic disease, the consequences of chemotherapy or other circumstances, such as haemorrage and/or surgery. Transfusion becomes indispensable to prevent the side effects of anemia, such as hypoxia, palpitations, tachycardia, cardiac ischemia and fatigue. However, frequent transfusions can cause several acute problems such as hemolysis, anaphylactic shock and septic shock but also chronic problems such as iron overload (hemochromatosis), alloimmunisation and metabolic disturbances. Each of these complications can produce serious consequences and could even be sometimes fatal. Therefore we should recognise, prevent and if necessary treat all these hazards. Our article emphasises the potential chronic problems. For hemochromatosis, an iron chelator (deferoxamine) should be administered. In the presence of allo-immunisation the more compatible ABO blood group must be chosen and blood products be eliminated by filtration, when there has been blood reaction. When an allo-graft of hematopoitic tissues is considered an irradiation of blood products is necessary. Research is being carried out to develop substitute products for transfusion (haemoglobine solutions) or molecules acting on the syntheses of haemoglobine (butyrate arginine). The efficacy of erythropoitine, (EPO) is well recognised for stimulation of haemoglobine syntheses in renal failure and oncology.