51 patients undergoing elective total knee and hip arthroplasty were randomized to receive thromboprophylaxis with either subcutaneous injection of fondaparinux (2.5 mg) or aspirin (325 mg) plus compression stockings. The purpose of this study was to determine whether changes in thrombogenicity can be measured with blood coagulation factors and platelet counts. Analysis of blood drawn on postoperative day 2 revealed significant increases in collagen-induced platelet aggregation and TIP-FCS in the fondaparinux group whereas no increases were observed with the aspirin group.