The package insert of the antithrombotic agent warfarin warns users of its interaction with azole antifungals. However, information on the frequency or degree of these interactions is limited. In particular, the time to onset of azole-mediated prothrombin time prolongation, expressed as the international normalized ratio (INR), is poorly characterized. Therefore, we retrospectively examined the INR in 29 patients administered warfarin with fluconazole (FLCZ), voriconazole (VRCZ), or itraconazole (ITCZ). INRs in 18 patients taking FLCZ and in 5 patients taking VRCZ significantly increased from 1.40 to 2.94 and from 1.95 to 2.89, respectively. The warfarin sensitivity index (WSI), calculated as INR/daily warfarin dose, also significantly increased from 1.06 to 1.89 with FLCZ and showed an upward trend from 1.13 to 2.23 with VRCZ. ITCZ had no influence on the INR or WSI in 6 patients. The INRs observed when warfarin was coadministered with azoles ( Y ) correlated significantly with those observed in the absence of azoles ( X ): FLCZ, Y =4.94 X −3.96, r ²=0.80; VRCZ, Y =2.13 X −1.27, r ²=0.93. Moreover, in all 8 patients with closely monitored INRs, the WSI increased within 1 week of FLCZ or VRCZ coadministration. In conclusion, FLCZ and VRCZ augmented the anticoagulant activity of warfarin. The INR should be closely monitored within 1 week of initiating FLCZ or VRCZ coadministration with warfarin, especially in patients with high INRs.