Thrombotest® results expressed in international normalized ratio (INR) values, obtained in 108 patients on oral anticoagulant treatment, were compared with prothrombin time (PT) results with a recombinant thromboplastin. The former results were obtained on an Amelung coagulometer, the latter on a photo-optical instrument. Using the Thrombotest method, performed within 2 h after sampling as the reference method, a first group of 63 patients had an INR value between 2 and 4. This group was considered as adequately anticoagulated and served as a true positive population in further analysis. The remaining 45 patients (true-negative group) had an INR value below 2 or higher than 4 and could thus be considered as inadequately anticoagulated. Using these definitions, a sensitivity of 86% and a specificity of 96% could be calculated for the PT with the recombinant thromboplastin. All tests from patients on oral anticoagulant treatment were also performed after 24 h storage of the blood or plasma samples at room temperature. When we compared the reference Thrombotest results with those of the late Thrombotest and the late PT recombinant thromboplastin, sensitivities of 86 and 86% as well as specificities of 91 and 96% were found, respectively. In conclusion, PT with a recombinant thromboplastin on a photo-optical instrument, even after prolonged storage of the plasma samples at room temperature, can be considered as suitable for oral anticoagulation control.