Following careful consideration of physiopathological modifications during haemodilution, reference is made to experience with patients submitted to major surgery, mainly chest and vascular. The technique employed is preoperative normovolaemic moderate acute haemodilution. Reference is also made to the criteria for selecting patients, and the evaluation of clinical and haematochemical parameters used in applying this technique. The experience is judged to be positive and haemodilution is regarded as a useful tool for reducing the use of homologous conserved blood and for preventing the risks connected with it.