Phenprocoumon, whose elimination half-time is 144 hours, has been the traditional oral anticoagulant of choice in Europe. However, today's most widely used drug is warfarin, whose elimination half-time is 40 hours. This study aims to evaluate a method for safe transition from phenprocoumon to warfarin, which is sometimes required. Hence, the large difference in their elimination rates may on occasion lead to serious overdosage upon transition from one drug to the other. According to average equipotent doses, a stepwise increase in warfarin dose was calculated based on the elimination half-times of the two drugs. The dosage scheme was subsequently tested in a pilot study including 35 patients. The conversion scheme was then adjusted based on the results from the pilot study. The new scheme was tested in 69 patients. The transition factor was 2.3, which implies that equipotency was achieved when the warfarin dose was 2.3 times larger than the phenprocoumon dose (in mg). This scheme proved optimal for 75% of the patients. However, the dose had to be adjusted individually in the remaining 25% of the patients to a level corresponding to the measured international normalised ratios. No patients experienced haemorrhages or thromboembolic complications during the period of changeover. In conclusion, the proposed scheme for changing medication from phenprocoumon to warfarin is safe and convenient.