STUDY DESIGN The first case of intraspinal synovial cyst communicating with the C1-C2 facet joints and subarachnoid space associated with rheumatoid atlantoaxial instability is presented. OBJECTIVES To describe the diagnosis and successful treatment of a synovial cyst of the C1-C2 junction in a patient with rheumatoid arthritis and atlantoaxial instability. SUMMARY OF BACKGROUND DATA Intraspinal synovial cysts of the C1-C2 junction are extremely rare. Neither association with rheumatoid atlantoaxial instability nor communication with the C1-C2 facet joints and the subarachnoid space has been previously reported in a synovial cyst of the C1-C2 junction. MATERIALS AND METHODS The clinical and radiologic features of a 71-year-old woman with a symptomatic synovial cyst of the C1-C2 junction with rheumatoid atlantoaxial instability are detailed. Posterior atlantoaxial fusion alone was performed. RESULTS Preoperative magnetic resonance images demonstrated a large cystic mass around the dens, compressing the spinal cord. The mass was of low signal intensity on T1-weighted images, was of high signal intensity on T2-weighted images, and was enhanced marginally with gadolinium-DTPA on T1-weighted images. Computed tomograms performed after myelography disclosed the cyst around the dens communicating with the subarachnoid space and the C1-C2 facet joints. Spontaneous regression of the cyst was identified on the magnetic resonance images 3 months after surgery. One year after surgery, myelopathic symptoms were improved. CONCLUSION An intraspinal cyst communicating with the C1-C2 facet joints and subarachnoid space associated with rheumatoid atlantoaxial instability was reported. Computed tomograms after myelography were useful in delineating the relationships among the synovial cyst, facet joints, and the subarachnoid space. Spontaneous regression of the cyst and clinical improvement were achieved by C1-C2 posterior fusion alone.