The clinical use of the direct inhibitors of thrombin requires a reliable test to monitor the treatment and to predict the hemorragic risk. The activated partial thromboplastin time (APTT) is the most common test used to monitor treatment with unfractionated heparin. Thus APTT has been first chosen to follow patients treated with direct thrombin inhibitors, but studies have shown that it was probably not the most appropriate test. Indeed, APTT values were not well correlated with the dose administered and were dependent on the type of the thrombin inhibitor used and on the APTT reagent. The ecarin clotting time (ECT), which converts prothrombin into meizothrombin has been then tested and seemed to be a better test. In vitro studies have shown a good correlation between ECT and the different concentrations of thrombin inhibitors. Furthermore, the ECT in contrast to APTT is not sensitive to heparin or oral anticoagulant and interindividual variations are low with ECT. ECT which is a reliable test and is easy to perform seems to be a more appropriate test to monitor treatment with direct thrombin inhibitors but further studies are needed to validate its use in a clinical setting.