Twenty normal human subjects and 14 patients with upper cervical spine pathology were studied with axial high-field magnetic resonance (MR) imaging to examine the transverse atlantal ligament. Gradient-echo MR imaging pulse sequences provided reliable visualization of the transverse ligament, which exhibited low signal intensity and extended behind the dens between the medial portions of the lateral masses of C-1. The MR imaging characteristics of the transverse ligament were verified in clinical studies and in postmortem specimens. The clinical MR examinations defined 27 normal ligaments, three ligament disruptions, and four stretched rheumatoid ligaments. Atlantoaxial instability associated with transverse ligament rupture or ligamentous laxity required internal fixation. In contrast, fractures of C-1 or C-2 or atlantoaxial rotatory dislocations associated with an intact transverse ligament healed without instability or nonunion. The transverse ligament is the primary stabilizing component of C-1. The treatment of atlantoaxial instability has previously been based on criteria drawn from computerized tomography or plain radiographic studies, which only indirectly assess the probability of rupture of the transverse ligament. It is concluded that MR imaging accurately depicts the anatomical integrity of the transverse ligament. After transverse ligament failure, the remaining ligaments of the craniovertebral junction are inadequate to maintain stability. The presence of ligament disruption should be considered as a criterion for early fusion.