Background:This study evaluated the rates of new lesions on diffusion-weighted images (DWIs) of magnetic resonance imaging (MRI) and hemorrhagic transformation (HT) during 2 weeks after acute ischemic stroke (AIS) in patients with atrial fibrillation (Af) who were given one of the non-vitamin K antagonist oral anticoagulants (NOACs); this was then compared with those who were given warfarin.Methods and Results:Consecutive AIS patients with Af were enrolled between January 2008 and June 2013, and those selected were patients who had a MRI that included DWIs both on admission and after 2 weeks, and those given only wafrarin (warfarin group) or only one of the NOACs (NOAC group) within 2 weeks of admission. Of all 257 enrolled patients, 50 patients were selected for the NOAC group (median age of 80.0 years) and 125 patients for the warfarin group (median age of 80.0 years). Both NOAC and warfarin were started at a median of the second day after admission. There was no significant difference in the rates of new lesions on DWIs (26.0% vs. 28.0%, P=0.7888) and HT (30.0% vs. 39.2%, P=0.2536) between the NOAC and warfarin groups. The NOAC group had a lower rate of concomitant use of heparin (44.0% vs. 92.8%, P<0.0001) than the warfarin group.Conclusions:This study suggests that NOACs are suitable for AIS patients with Af, perhaps even better than warfarin, given their simplicity. (Circ J 2015; 79: 862–866)