Non-progressive juvenile spinal muscular atrophy of the distal upper limb (Hirayama's disease) occurs predominantly in men in the second decade with unilateral weakness of the fingers and hand, and atrophy of the hand and forearm. The symptoms generally progress for 3-4 years after onset and then stop. Since the second decade is a very important period starting one's career, early stopping of the progression and possibly the improvement are essential. On the bases of the neuroradiological studies, a forward displacement of the posterior wall of the dural canal at the lower cervical level on neck flexion is presumed to be the main pathogenetic mechanism in this disease. On neck flexion, the spinal cord is compressed anteroposteriorly at C7 and C8 segmental levels. Pathological studies suggested that chronic microcirculatory disturbances in the spinal cord as the result of repeated flexion of the neck might produce necrosis of the anterior horns. We used a cervical collar for the prevention of neck flexion in 14 patients in the early stage and compared the results with those of 18 patients who had natural courses. In the therapy group, muscle weakness of the affected hand improved in 5 patients and the progression of the disease was stopped in 8 patients with the duration of 1.8 +/- 1.2 years, whereas the duration of progression of the natural course group was 2.9 +/- 1.6 years. Only one patient who could not tolerate the use of collar showed a slight progression. These results suggest that the cervical collar is a useful therapeutic tool in this disease.