Anticoagulants of the cumarin-type (warfarin, phenprocoumon, and acenocoumarol) are drugs for the long-term treatment and prevention of thromboembolic disorders. Because of their narrow therapeutic range, many patients have bleedings of variable severity or have recurrent thrombotic events. For this reason, the study of the pharmacokinetic parameters of phenprocoumon (PPC), considering its influence on blood clotting factors, is of high interest. The elimination kinetics of PPC, its interaction with phytomenadion (vitamin K), and the pharmacokinetic behavior of the anticoagulant under steady-state conditions have been investigated in studies with healthy volunteers and patients taking anticoagulants. The maintenance dose and the plasma levels of PPC were correlated with prothrombin time (PT) in 89 patients treated with PPC. Varying parameters in each patient (e.g., elimination kinetics of PPC, activity of the cumarin-dependent blood-clotting factors, endogenous phytomenadion stores), render it impossible to use a different means of monitoring than that of PT determination.