As the cause of brachial plexopathy, an aneurysm of the subclavian artery is rare and mostly related to trauma. Early diagnosis and treatment is very important because the arterial aneurysm itself is life-threatening and nerve injury can be reversible in cases of early treatment. We report a patient with Behçet's disease having a right brachial plexopathy caused by a nontraumatic aneurysm of the right subclavian artery.