Argatroban, which is a thrombin inhibitor, has an indication as a treatment in the acute phase on atherosclerotic ischemic stroke in Japan. Howeve, in cardioembolic stroke, argatroban is considered to be contraindicated with the side effect of hemorrhage, though there is no clear clinical evidence to show that argatroban increases hemorrhagic compared with heparin. The efficacy of anticoagulant treatment with argatroban on cardioembolic stroke was evaluated retrospectively in this study. We identified 3,113 patients from the Japan Standard Stroke Registry Study who had had a cardioembolic ischemic stroke. We excluded patients with the anti-platelet treatment or the combination therapy of anticoagulation. Our analyses are therefore based on a cohort of 2,529 patients who were treated either with heparin, and argatroban, or with no anti-coagulation treatment. With multivariable regression, hemorrhagic it was shown that hemorrhage was significantly reduced in heparin and argatroban treatments in the patients with mild severity. There was no significant difference in the recurrence of ischemic stroke between the treatments. Both argatroban and heparin showed dramatic improvement compared with the no treatment standard, but only heparin achieved statistical significance for mortality and change in NIHSS score (admission to discharge) in the moderate stroke subgroup [NIHSS 11-22]. Both heparin and argatroban [more so than heparin alone] have a significantly reduced mortality risk. From the present study, it is suggested that argatroban may be useful on cardioembolic stroke, increasing the improvement of recovery of stroke severity without increasing the risk of hemorrhage. Further prospective studies are awaited for evaluating better the efficacy of argatroban on cardioembolic stroke.