A 56-year-old woman noticed non-progressive weakness in the proximal part of the right upper extremity from her childhood. At the age of 37 years, she was diagnosed as "spinal muscular atrophy" by an orthopedic surgeon. At the age of 56, neurological examinations revealed muscular atrophy and weakness confined to the right deltoid, biceps brachii and brachioradialis together with minor sensory disturbance in the lateral side of the right shoulder and forearm without pyramidal sign in the lower extremities. The neurological features of this case differed from those of juvenile type of distal and segmental muscular atrophy of upper extremities in distribution of muscular atrophy, and simulated those of cervical spondylotic amyotrophy. Myelography demonstrated root avulsion of the right C5 and C6 roots. CT myelography revealed traumatic meningocele. Therefore a clinical diagnosis of cervical root avulsion resulting from unrecognized birth injury was made. The reason of motor dominant pictures of this case may be vulnerability of the anterior nerve roots to traction injury.