The aim of this work was to investigate whether the thrombin activity is related to the degree of anticoagulation induced by oral anticoagulants. Moreover, we tried to detect an optimal anticoagulation range at which the lowest possible thrombin activity can be reached. We investigated 28 patients (19 women and 9 men, mean age 54 ± 9 years). Anticoagulation had been induced by acenocoumarol for at least 1 year before the beginning of this study. The degree of anticoagulation was monitored by the thrombotest coagulation method. The therapeutic range was 5–13%. The thrombin activity was measured by means of the fibrinopeptide A radioimmunological assay. In 15, 7, and 6 of the patients, thrombotest and fibrinopeptide A were carried out twice, once, and three times, respectively. Our results show first of all a significant positive relationship between thrombotest and fibrinopeptide A (p < 0.001). Once this result was obtained, we tried to improve our identification of the behaviour of the thrombin activity in relation to the degree of anticoagulation assessed by thrombotest. For this purpose we employed a third-degree polynomial regression analysis which showed a better fit of the data. Since the curve became steeper from about 10% thrombotest levels, we divided the FPA values on the basis of thrombotest ranges. FPA values for the 14- to 25% thrombotest range were significantly different from those in the thrombotest range of 4–10 %. Moreover, FPA levels in the 11 to 13 % thrombotest range were significantly different from those in the thrombotest range of 4–10%. Our results suggest that a significant decrease in thrombin activity may be achieved only with a deep anticoagulation.