Described in this article is diagnosis and treatment of mixed type of thoracic outlet syndrome (TOS) resulting from scalene muscle hypertrophy. Many diseases should be considered in the differential diagnosis to definitively diagnose this syndrome, and it is necessary to ask about activities of daily life, in addition to adding provocative tests to physical examination and examining electrophysiological and imaging results. Once diagnosed, even if complaints decrease with conservative treatment, surgery may be required. However, in patients diagnosed late, despite decrease in complaints after surgery, neurological deficit may remain.