Giant cell arteritis (GCA) and polymyalgia rheumatica (PMR) are closely related disorders found in older patients, and vasculitis has been proposed as a part of the pathogenesis of PMR. We describe a female patient with PMR plus aortitis, both of which were well controlled on maintenance steroid therapy. Six months after the onset of her condition, however, she suddenly presented with chest pain. A diagnosis of dissecting aortic aneurysm was confirmed, and the aorta was successfully resected. Histology revealed infiltration of mononuclear cells including giant cells around the vaso vasorum with disruption of elastic lamina of the resected aorta. PMR or GCA may indicate an increased risk for aortic dissection in patients with normal erythrocyte sedimentation rate or C-reactive protein, and prompt recognition and therapy, not only during the active disease but also after symptoms of PMR have resolved, are needed.