The significance of acquired antithrombin III (AT III) deficiency must be interpreted in close relation to the underlying disease process. In patients with acute or chronic liver impairment, the AT III activity is related to a decrease of procoagulatory factors, whereas, in protein loss syndromes such as nephrotic syndrome, the AT III indicates an increased risk of thromboembolic events. The effect of oral contraceptives (OC) on AT III levels in young healthy females (n = 30) was determined prospectively. AT III decreases during OC usage could not be related to the estrogen content of the examined oral contraceptives, and there was no parallel decrease of AT III activity and concentration in each type of OC. In a prospective study, the extent of AT III decrease was determined in patients undergoing cardiopulmonary bypass operations (CPB) receiving different anticoagulant schedules during extracorporeal circulation (n = 49). There was no significant influence on the effectiveness of anticoagulation by the observed AT III decreases. AT III deficiency during CPB was primarily the result of hemodilution. However, the AT III kinetics were significantly influenced by the different protamin dosages and were not affected by the different heparin dosages. Correction of diminished AT III levels by substitution of AT III concentrates is beneficial in cases, in which an interruption of an enhanced coagulatory process such as disseminated intravascular coagulation is necessary or in patients requiring high dosage heparinization as in deep vein thrombosis. In those cases the quality of AT III correction correlates to the course of the disease. However, the potency of concentrates as well as the individual AT III recovery and half-life must be considered for an appropriate treatment with AT III substitution.