Thoracic outlet syndrome (TOS) is a combination of signs and symptoms caused by the compression of the vital neurovascular structure at the thoracic outlet region. It may stem from a number of abnormalities, including degenerative or bony disorders, trauma to cervical spine, fibromuscular bands, vascular abnormalities and spasm of the anterior scalene muscle. CPT (current perception threshold) is defined as the minimum amount of current applied transcutaneously that an individual consciously perceives. It enables quantification of the hyperesthesia that precedes progressive nerve impairment, as well as hypoesthetic conditions. We experienced a case of thoracic outlet syndrome caused by fibrosis of anterior scalene muscle. The patient was a 30 years old woman with a 3 years history of numbness on the ulnar side, progressive weakness and coldness of both hand, tiredness in the left arm, nocturnal pain in the left forearm, and pain in the left elbow, shoulder and neck. Conservative treatment, stellate ganglion block, cervical epidural block, anterior scalene block and previous operation, including both carpal tunnel release, provided no remarkable relief to the patient. A left scalenectomy and first rib resection were performed by transaxillary approach and left cervical root neurolysis was done. After surgery, we measured CPT using neurometer and found conditions worsening in the opposite arm. We performed the same procedure on right side, and followed by CPT measurement. This case suggests that CPT is a useful measurement of recovery and progression of TOS.