Thoracic outlet syndrome is a disorder caused by thoracic outlet compression of the brachial plexus and/or the subclavian vessels. The characteristics of thoracic outlet syndrome are highly variable. Objective tests, such as electrodiagnostic studies, are often unreliable in characterizing thoracic outlet syndrome. The existence of thoracic outlet syndrome as a discrete entity is controversial. Surgeons who accept the existence of thoracic outlet syndrome acknowledge that diagnosis is clinical. The variability and complexity of thoracic outlet syndrome lends itself to mistakes in both diagnosis and surgical treatment.