The study is a national multicenter prospective observational study, including 200 patients. The main purpose of this study is to explore in more detail the influence of genetic variability (CYP enzymes and vitamin K dependent proteins) and dietary vitamin K status on warfarin dosing, clinical effect and adverse events with emphasis on the initial phase of treatment. The hypothesis is that genetic variability concerning CYP enzymes and vitamin K dependent proteins predict dosing and adverse events during warfarin treatment. The main aim is to individualize warfarin therapy and establish a treatment algorithm based on genotype and dietary vitamin K status to make the anticoagulation therapy with warfarin more secure.