A 65-year-old woman experienced progressive intrinsic muscle wasting on the right hand over a period of 7 years. The distribution of muscular atrophy and weakness was consistent with the area innervated by the right C8 and Th1 nerve roots. Neurophysiological examination suggested a right lower trunk lesion. An elongated right transverse process of the C7 vertebra and an aberrant subclavian artery were detected on computed tomography images, and the right lower trunk of the brachial plexus appeared to be lifted upward on magnetic resonance images. The patient was diagnosed with true neurogenic thoracic outlet syndrome. A fibrous band extending from the elongated transverse process was found during surgery, and symptoms did not progress further after resection of the band. True neurogenic thoracic outlet syndrome can cause monomelic amyotrophy, and localized neuroimaging and detailed neurophysiological examination were useful for diagnosis.