In a clinical study the effectiveness of aggregation limiters (Dextran and acetyl-salicylic acid) and preoperational normovolaemic haemodilution as prophylaxis for deep vein thrombosis (D.V.T.) was assessed on the basis of experimental work in 404 surgical patients. Our patients were monitored by means of the fibrinogen test and ascending phlebography. Our results show that haemodilution is a very effective preventive for D.V.T. Therefore, moderate normovolaemic haemodilution can be applied as an effective prophylactic measure for postoperative deep vein thrombosis.