Background and Purpose— The prevalence of long-term oral anticoagulant therapy is rising. Treatment options for patients who have an ischemic stroke under oral anticoagulant therapy are limited, and clinical data on the risk of hemorrhagic transformation (HT) in this condition are scarce. We therefore aimed to establish a mouse model of ischemic stroke occurring during oral anticoagulant therapy to assess the frequency and characteristics of HT. Methods— C57BL/6 mice (n=59) were pretreated with warfarin. Untreated mice (n=32) served as controls. We performed a 3-hour transient filament occlusion of the right middle cerebral artery. In a first set of animals, ischemic lesion size and HT were evaluated macroscopically at 24 hours after middle cerebral artery occlusion. In a second set of mice, quantitative analysis of HT was performed at different time points after middle cerebral artery occlusion and in animals with different international normalized ratio levels using a photometric hemoglobin assay. Results— Oral anticoagulant therapy at the onset of ischemia led to HT in all anticoagulated mice, whereas only 14% of the control mice showed HT. Mean HT blood volume 24 hours after middle cerebral artery occlusion was 0.3±0.4 μL in controls, 4.2±1.7 μL in mice anticoagulated to a mean international normalized ratio of 1.9±0.5 ( P <0.05 versus controls), and 5.2±2.7 μL in mice with an international normalized ratio of 2.9±0.9 ( P <0.001 versus controls). Anticoagulated mice euthanized at the time point of reperfusion had less HT than mice euthanized after 21 hours of reperfusion (1.6±0.5 μL versus 5.9±3.6 μL, P <0.05). Conclusions— We present a mouse model of ischemic stroke occurring during oral anticoagulant therapy. Warfarin pretreatment dramatically increases the risk of HT 24 hours after middle cerebral artery occlusion. Reperfusion injury seems to be a critical component in this condition.