The diagnosis, cause, and treatment of thoracic outlet syndrome (TOS) are challenging, and the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides) gives no specific instructions about rating impairments associated with this disorder. TOS is a group of symptoms and signs, including pain, weakness, paresthesias, numbness, swelling, and/or coldness arising at the base of the neck or superior chest but involving the upper limbs. Causes include compression of the brachial plexus (neurogenic TOS) or, less frequently, subclavian vein and/or artery (vascular TOS). Provocative tests for TOS attempt to induce symptoms and/or signs by compressing the subclavian artery or vein or brachial plexus, but palpitation of the radial pulse in various extremity positions is of limited diagnostic value. Diagnostic studies may include radiographs of the cervical spine, upper chest, and/or shoulder; MRI scans of the thoracic outlet; arteriography or venography, and electrodiagnostic testing. The differential diagnosis for TOS is broad and includes cervical radiculopathy, superior sulcus tumor, other peripheral nerve entrapment syndromes, complex regional pain syndrome, and psychiatric disease. The evaluator must determine if the symptoms, physical findings, and diagnostic study results support the diagnosis; the report should list the rating options considered, discuss the rationale for selecting the method(s) used, and explain how the percentage was calculated.