In this study the warfarin level in the plasma of 60 patients receiving different dosage regimens of warfarin were determined by HPLC. The corresponding prothrombin times (PT) were also measured. The steady state plasma level of warfarin in 10 healthy volunteers was 900.7+/-158.21 ng/ml. A mean dose of 3.79+/-1.02 mg of warfarin corresponded to a therapeutic level of 1193.81+/-300.35 ng/ml with a mean PT of 20.13+/-0.69 s. The results of this study suggest that Iranian subjects are more sensitive to warfarin than North American and European subjects, but have responses very similar to those of Asians. Dietary, ethnic and geographical factors as well as differences in drug dispensition may be the cause of the observed dosage variability of warfarin.