A 61-year-old woman without rheumatoid arthritis (RA) was admitted with atlantoaxial dislocation (AAD) and a retroodontoid mass at the craniovertebral junction manifesting as a 1-year history of numbness and mild weakness of the right upper extremity. Computed tomography and magnetic resonance (MR) imaging showed AAD and a mass at the craniovertebral junction. She had no past history of RA or trauma in the head and neck. She underwent surgery to obtain the histological diagnosis of the mass and to improve AAD-induced instability. The lesion was approached through the right transcondylar fossa approach with C-1 laminectomy. Intraoperative pathological examination showed cicatrizing collagen fibers and no obvious tumor cells. After partial removal of the lesion, the AAD was fixed with the posterior approach. The symptoms subsided soon after surgery and the mass decreased on MR images taken 3 months after surgery. If a pseudotumor is suspected based on the preoperative radiological investigation in a non-RA patient with AAD and the symptoms are not progressive, stabilization can be expected to induce spontaneous regression without urgent direct excision of the mass.