We describe herein a patient who developed a dissection of the ascending aorta in the setting of IgG4-related systemic disease, linking IgG4-related systemic disease with a newly-recognized subset of noninfectious aortitis. At the time of aortic surgery, a transmural lymphoplasmacytic infiltrate was detected in the patient's aorta, with a principal focus of inflammation within the media. Immunohistochemical studies demonstrated that >50% of the plasma cells in the lesion stained for IgG4. By in situ hybridization, the plasma cells showed polytypic staining for kappa and lambda light chains, consistent with a polyclonal plasma cell infiltrate. Serologic evaluation revealed that the patient's IgG4 levels were elevated nearly 10-fold. Four years before aortic surgery, the patient had undergone a mediastinal lymph node biopsy. Reexamination of the lymph node revealed features consistent with IgG4-related systemic disease, which had not been recognized at the time of the original biopsy. Glucocorticoid therapy for the IgG4-related systemic disease yielded a prompt response. Recognition that IgG4-related systemic disease can involve the ascending as well as the descending abdominal aorta indicates the need for a change in the way idiopathic aortitis is regarded. This case offers new potential considerations for short- and long-term management of noninfectious aortitis, because of the frequent good response of IgG4-related systemic disease to glucocorticoid treatment without additional therapy. Treatment of the aortitis may prevent progression of the IgG4-related systemic disease to involvement of other organs. IgG4-related systemic disease should be considered in all patients with aortitis judged to be of unknown etiology.