Oral anticoagulant therapy has been practiced for 40 years but we still do not have definitive evidence of its value. Early clinical trials are now discounted due to the varying quality and lack of standardisation of the thromboplastins then available. There was also no statistical evidence that therapy was adequately maintained. We now have sensitive and reliable thromboplastins and there is an international standardisation of calibration and of reporting prothrombin time. With therapeutic quality control of oral anticoagulation and the so treated patients clinical assessed, we might hope that definitive evidence of the value of this therapy would result.