A 65-year-old woman with a history of chronic atrial fibrillation was admitted for treatment of progressively worsening pain in the right forearm, which had begun several days before presentation. She did not have a palpable right brachial pulse, but her axillary pulse was palpable. We suspected acute thromboembolism of the brachial artery, and tried emergency thrombectomy via the brachial artery. However, her brachial artery was completely occluded with white organized thrombi, which was the cause of the chronic occlusion. Vasodilative drugs were administered, but her symptoms remained. Preoperative angiography showed the right brachial artery to be occluded and the ulnar artery could be partially visualized via the collateral arteries. We performed bypass grafting between the right axillary and ulnar artery, using an autologous saphenous vein graft. Her symptoms improved, and postoperative angiography revealed the bypass graft to be patent. The possibility of acutely worsening chronic ischemia should be considered, even if symptoms indicate acute thromboembolism of the extremities.