The authors report a case of successful surgical treatment of atlanto-axial dislocation (AAD) secondary to athetoid cerebral palsy. A 61-year-old woman was admitted to our hospital in July 1993 complaining of progressive weakness in the right upper extremity and gait disturbance. She had been suffering from athetoid movements of her face, neck and arms due to cerebral palsy. Neurological examinations on admission revealed down-beat nystagmus on downward gazing, motor weakness of extremities, pallhypesthesia, hyperreflexia exaggerated in the right side and bilateral positive pathological reflexes. Lateral tomogram of the upper cervical spine demonstrated instability of the atlanto-axial joint, increased atlanto-dental interval (ADI) by 5.5 mm (in flexion), and narrowed canal at C1 level. Myelogram showed narrowed dural sac and angulation of the spinal cord at C1 level. A halo vest was applied two days before operation for reduction of the atlanto-axial junction and external fixation. She underwent posterior internal fixation using a Hartshill Ransford Loop combined with posterior decompression. This loop was secured to the occiput, C1, C2 and C3 by sublaminar wiring, and foramen magnum decompression and laminectomy of C1 were performed. Postoperative course was uneventful. Postoperative plain X-ray film, tomogram and computed tomography demonstrated good fixation (ADI was 2.5 mm) and excellent stability. There has been no problem during 6 months since the operation. It is known that involuntary movements in patients with athetoid cerebral palsy sometimes cause cervical spondylosis (especially at C3/4 and C4/5 level). Recently, AAD due to athetoid cerebral palsy has been reported. Almost every case of AAD secondary to athetoid cerebral palsy is combined with incompetence of the odontoid process.(ABSTRACT TRUNCATED AT 250 WORDS)