A 94 year-old Japanese woman with renal dysfunction was admitted to our hospital for congestive heart failure caused by atrial fibrillation with rapid ventricular response. Considering the risk of stroke, warfarin was commenced. However, she developed shock following brachial and retroperitoneal hemorrhage 4 days after starting warfarin despite not being over-anticoagulated. She recovered after receiving blood transfusion and intravenous vitamin K2. Bleeding during warfarin administration occurs more frequently in older individuals with lower glomerular filtration rates, especially within the first 30 days of warfarin treatment. Physicians should therefore check for unexpected bleeding after commencing warfarin and be prepared to reverse anticoagulation.