Aortitis syndrome is a chronic vasculitis that leads to arterial wall thickening and stiffening in large elastic arteries. However, there are no established markers for assessing arterial stiffening in aortitis syndrome. The cardio-ankle vascular index (CAVI) has recently been utilized to assess arterial stiffening that is associated with atherosclerosis-related diseases. We hypothesized that CAVI can be applicable for assessing alterations in arterial stiffness during immunosuppressive therapy for aortitis syndrome. A 69-year-old woman with a 2-month history of recurrent fever, fatigue, and malaise, showed intense 18F-fluorodeoxyglucose (18F-FDG) uptake in the thoracic aorta and common carotid arteries in 18F-FDG-positron emission tomography. These clinical and imaging findings resulted in the diagnosis of aortitis syndrome. The patient also showed the elevated CAVIs on both sides (right, 10.3; left, 10.4) (normal value for her age, 9.1 +/- 0.8), indicating the arterial stiffness due to aortitis syndrome. The patient was treated for 34 weeks with immunosuppressive therapy, which included oral prednisolone and methotrexate. C-reactive protein (from 4.24 to 0.49 mg/dL) and immunoglobulin G (from 2,627 to 1,524 mg/dL) were decreased by 7 weeks after initiation of the treatment. The decrease in these inflammatory parameters suggests the effectiveness of the immunosuppressive therapy. In addition, after the 34-week treatment, the CAVIs on both sides (right, 9.3; left, 9.2) were within the normal range. These data indicate that the immunosuppressive therapy ameliorates the degree of arterial stiffness. In conclusion, CAVI may be a promising marker for evaluating the effectiveness of immunosuppressive therapy in patients with aortitis syndrome.