Here we report a case of true neurogenic thoracic outlet syndrome (TN-TOS) with serial nerve conduction studies over 13 years. A 65-year-old woman presented with atrophy of the right thenar muscle, and mild numbness in the medial forearm from 25 years ago. Nerve conduction studies revealed sever reduction of the amplitude of the right median compound muscle action potential (CMAP), and loss of sensory nerve action potential of medial antebrachial cutaneous nerve. MR Angiography and ultrasound image showed superior and shallow shift of right subclavian artery. We diagnosed her with the TN-TOS, then surgically explored the brachial plexus with the supraclavicular approach. We found superior shift of the subclavian artery, just medial of upper trunk, and an aberrant muscle slip below the lower trunk, possibly entrapping it between the scalenus anterior muscle, then cut it to decompress the trunk. Her symptoms gradually improved, however disability of the thumb opposition remains for 2 years after surgery. In nerve conduction studies 13 to 3 years before, the right median CMAP was preserved although temporal dispersion was already observed. Early definitive diagnosis and surgical decompression for the TN-TOS should be done before the progress of axonal degeneration.