Requirements of donor blood, bleeding and platelet counts were investigated during moderate and extreme haemodilution in patients undergoing aortic valve replecement. A preliminary study of 60 patients, divided in two groups of 30 patients each, was first made. A moderate haemodilution was used in one group and an extreme haemodilution with the aid of autologous transfusion was utilized in the other. As the results were interesting and encouraging, a randomized study was performed. Thirty patients, divided in two groups of 15 patients each, were investigated. The degree of dilution was essentially similar to that of the corresponding moderate and extreme haemodilution groups in the preliminary study. An average reduction of 71% in the requirement of donor blood and of 47% in postoperative chest tube drainage was found in the patients in the extreme dilution group. The circulating platelet counts fell less during and after the operation in this group than in the moderately diluted patients. After adjustment for the dilution, the platelet counts fell to about 60% in the moderate dilution group during the perfusion, but rose to about 130% in the extremely diluted patients. These findings are discussed.