Blood and plasma viscosity has been controlled in a group of patients undergone to aorto-iliac reconstruction and in a group of control after thyroidectomy, cholecystectomy, and hernioplasty. The hemodilution induced by intraoperative infusion in the vascular reconstruction produced an important decrease of hematic and plasmatic viscosity which lasted for several days after the operation. Removing the hemodilution effect by a mathematical correction of the viscosity measured values to a standard haematocrit, it has demonstrated as surgical operation, apart from its entity, promoted an increase of the viscosity which persisted long in the postoperative course. For what it concerns the risk of postoperative thrombosis from one side protective effect of hemodilution is confirmed, from the other, in absence of the hemodilution, it would be useful to continue the antithrombotic prophylaxis longer the perioperative time as usual.