Thromboembolic complications of prothrombin complex concentrate (PCC) therapy were first reported by Kasper in 1973 [N Engl J Med 1973;289:160]. The following contaminants were discussed as possible contributors to the thrombogenicity risk: the presence of other zymogens in PCCs, the presence of activated factor IX or activated factor X, or the presence of phospholipids from platelets resulting from insufficient centrifugation of the donor plasma. Activated factor IX is now accepted as a major causative factor. After numerous additional reports of thromboembolic complications in patients treated with PCCs, the International Society on Thrombosis and Haemostasis (ISTH) Scientific and Standardization Committee (SSC) Thrombogenicity Registry was established in 1988. Lusher collected 72 cases worldwide for the SSC/ ISTH in 1988 and 1989. Thromboembolic complications and myocardial infarctions, however, continue to be serious problems associated with PCC therapy – even in the 1990s – underscoring the urgent need for high-purity factor IX products. Results from several studies by Mannucci, Bauer, and other authors demonstrated that patients treated with high-purity products developed no activation of prothrombin or thrombin, as indicated by appearance of thrombogenicity markers such as fibrinopeptide A and the amino-terminal fragments of prothrombin (F1+2). Other authors demonstrated that in patients at high risk for thrombotic complications, particularly those with liver disease, or postsurgery, or in those requiring repeated treatments, high-purity concentrates appear to be safe, regarding both thrombosis and risk of virus transmission.