Intraoperative blood collection prior to cardiopulmonary bypass (CPB) is a common procedure in cardiac surgery. Its aims are to obtain fresh whole blood to be infused after CPB, to use autologous blood in the prime or to avoid too high levels of haematocrit (HCT) during CPB. Calculations to determine the amount of blood to be collected are generally based on theoretical assumptions. In this paper we have set up and controlled, on a clinical basis, a nomogram to be used in order to obtain a standard pre-CPB HCT equal to 38%, using a moderately hypervolaemic haemodilution based on blood collection simultaneous with fluid replacement. Thirty adult patients were evaluated in order to settle the nomogram; subsequently, the nomogram validity was checked on 100 adult patients. Statistical analysis revealed the good clinical usefulness of this instrument.