Antithrombin III is an important element of the endogenous thrombosis protection system. Congenital Anti-thrombin III deficiencies are associated with increased incidence of thrombo-embolisms. If Antithrombin III is determined at the time of operation, postoperative deficiencies resulting from an increased turnover rate can be identified. Determination of Antithrombin III is not suitable for prognosticating the occurrence of thrombo-embolic complications in isolated cases. Antithrombin III can be administered in the form of fresh frozen plasma (FFP), though it is also available as Antithrombin III concentrate. So far no investigations have been carried out to establish whether it is possible to improve thrombosis protection with or without heparin prophylaxis by concentrated Antithrombin III substitution.