True neurogenic thoracic outlet syndrome (NTOS) is an extremely rare condition, probably occurring with an incidence of one per million. It is often misdiagnosed as carpal tunnel syndrome or benign focal amyotrophy, and careful clinical examination, electrodiagnostic studies, and imaging studies are essential for the correct diagnosis. Here, we report a 23-year-old woman with slowly progressive weakness and atrophy of abductor pollicis brevis who were found to have NTOS caused by cervical rib.