Little study has been performed on the effect of vitamin K intake on the variability of warfarin's anticoagulant effects over long period of time. We estimated average vitamin K intake in the patients taking warfarin and evaluated its relation with the stability of anticoagulation effect. We estimated average daily vitamin K intake based on a three-day food diary in 66 patients taking warfarin regularly for ≥ one year and divided them into three groups of equal number according to vitamin K intake. Stability of anticoagulant effect was compared in these groups using the coefficient of variation (CV) of the prothrombin time expressed in international normalised ratio (INR) and the CV of warfarin doses. Median daily vitamin K intake was 161.3 μg/day (31.3 μg/day - 616.6 μg/day). CVs of both INR and warfarin doses were negatively and independently correlated with dietary vitamin K intake (r=-0.293, p=0.017 and r= -0.350, p=0.004, respectively). CV of INR was significantly different among three groups of vitamin K intake (p<0.05 in ANOVA). High vitamin K intake (>195.7 μg/day) group had lower CV of INR than the low intake (<126.5 μg/day) group (19.2 ± 8.96 % vs. 25.5 ± 8.61 %, p<0.05). CV of warfarin doses was also significantly different among the groups (p<0.05 in Jonckheere-Terpstra test). However, the significance of difference between high and low vitamin intake groups was marginal (p=0.046 in Mann-Whitney test). In conclusion, long-term anticoagulation effect of warfarin is more stable in the patients who take greater than a certain amount of dietary vitamin K.