During the last three years we have experienced 20 cases of atlanto-axial dislocation (AAD) of various types. The series consists of 10 cases of traumatic anterior pure AAD, 2 traumatic anterior AAD with dens fracture, 2 os odontoideum, 1 rheumatic AAD, 1 Jefferson's fracture, and 3 "rotatory dislocation" (1 pure traumatic rotatory dislocation and 2 rotation deformity after Wackenheim). All patients were studied by plain X-ray, tomography, myelography with water soluble contrast media and computed tomography (CT) scanning. All of the operated patients and a part of the non-operated patients were also examined by CT myelography (met. CT) 3 hours after myelography. A special attention was paid to the shape of the spinal cord and the thecal sac at the level of dislocation, in several positions of the head such as hyperextension, neutral position, flexion, lateral bending and right or left rotation. A met. CT could clearly demonstrate that the thecal sac and the spinal cord were compressed by the dens and/or posterior arch of the atlas in traumatic pure anterior AAD. In all cases of ADD with dens fracture or os odontoideum, however, the spinal cord was compressed not by the dens but by the posterior margin of body of the axis. The fractured dens always moved with anterior arch of the atlas. This finding was most clearly documented by met. CT in flexion. In some cases of AAD accompanying rheumatic changes or narrowing of the spinal canal, a posterior decompression with postero-lateral fusion is indicated. A transoral anterior fusion in hyperextended position of C1 and C2 together with the fractured dens, followed by a Halo vest fixation, is the treatment of choice in traumatic AAD with dens fracture and os odontoideum. A posterior C1 and C2 fixation using Urlich's metal plate is also recommended for anterior pure AAD. CT scanning, especially met, CT, is by far the better than the conventional radiologic examinations to determine the indication, approach and mode of operation in AAD.