In the Netherlands the prevention and treatment of thrombotic diseases by means of coumarin derivatives are carried out by “Thrombosis Services”, which cover a territory of 9 million inhabitants where they perform more than 2 million prothrombin estimations a year. A thrombosis service is an institute with the object to an appropriate treatment of patients with oral anticoagulants by means of:providing dosage and other advices by expert doctors attached to the service;venapuncture by skilled nurses at the patients home or at out-patient departments and standardized laboratory work. The Thrombotest of Owren is used for thelaboratory assay, aiming at therapeuticlevels of 5-10%. In a well organized thrombosisservice a score of 83% of thelaboratory results within the target limits are made. In 1971the thrombosis services were united in a federation whichset itself to guality control and improvement by means of standardization of techniques in laboratorycontrol, directivesfor dosage, instruction to patients, external quality control, etc. This highly privileged condition seems one of the reasons that the Netherlands do notsharethe negative attitude towards the efficiency of coumarin described in many other countries.