Newly diagnosed patients of both genders and at least 18 years old who need anticoagulant treatment with either acenocoumarol, phenprocoumon or warfarin within the low intensity INR range will be included in the trial. Main study parameters/endpoints: The % time within therapeutic INR range in the first 3 months of anticoagulation therapy. Nature and extent of the burden and risks associated with participation, benefit and group relatedness: Six extra blood samples are taken from each participant at the start of the study. Patients also have to attend 8 scheduled visits within the 3 months study period and are asked to fill in questionnaires. The genotype-guided dosing algorithm is anticipated to improve the accuracy of coumarin dosing and thus improve the safety and efficacy of anticoagulation therapy.