Objective
To study the incidence and degree of the cervical instability in the cerebral palsied patients and to investigate the cause of the high incidence of cervical myelopathy in these patients.

Method
The static and dynamic radiography of the cervical spine in the sagittal plane was performed in seventy-two patients with athetoid and spastic cerebral palsy and the incidence of spondylolisthesis, range of motion(by the Penning Method), sagittal diameter of the cervical canal, and posture of the cervical spine were evaluated.

Results
The incidence of the cervical spondylolisthesis was fifty percent with athetoid cerebral palsy and twenty-seven percents with spastic cerebral palsy. The incidence of spondylolisthesis was especially high at the level of C3/4 and C4/5. The excessive range of motion in flexion/extension by the sagittal plane was observed in 66.7% of athetoid patients and 53.3% of spastic patients, especially at the C2/3 and C3/4 levels. The abnormal curvature was noted in 66.6% of athetoid and spastic patients. C-curve and S-curve were more common in athetoid patients and straightening of the C-spine was more common in spastic type. A sagittal diameter of the cervical spinal canal was significantly decreased in patients with athetoid patients with C3/4 spondylolisthesis and/or abnormal curvature such as a C-curve or S-curve(P＜0.05). Height of the vertebral body was decreased in both athetoid and spastic patients.

Conclusion
The combination of a cervical instability and a narrow spinal canal predisposes the neurological progression to a cervical myelopathy in cerebral palsied patients.