Chronic inflammatory diseases, such as chronic polyarthritis or spondylarthritis ankylopoietica, can occasionally lead to vertebral instabilities of the occipitoatlantal or atlantoaxial level, requiring some form of stabilizing operation. By means of functional magnetic resonance imaging, i.e., with the cervical spine at its maximal range of flexion and extension, performed on 11 patients suffering from an instability at the above level, it was possible to demonstrate not only the extent of synovial tissue, but also how this sometimes excessive soft-tissue growth hinders an adequate reduction of the subluxated vertebrae. The importance of this investigation lies in the fact that the necessary operation can be planned appropriately. As such, three patients required a decompressing operation entailing resection of the posterior arch of the atlas, widening of the foremen magnum, or both.