['Eosinophilic angiocentric fibrosis (EAF) is an uncommon tumefactive lesion of the orbit and upper respiratory tract of unknown etiology. The condition is characterized histologically by concentric layers of fibrosis around small-caliber arteries and a mixed inflammatory infiltrate dominated by eosinophils. After the serendipitous observation of an elevated serum concentration of IgG4 in 1 patient with EAF, we investigated the hypothesis that EAF is an IgG4-related systemic disease.', 'We retrospectively identified 5 EAF cases from our files. Demographic, clinical, and serological data were reviewed, and the histologic features and tissue IgG4 staining patterns were examined on biopsies from each case.', 'Patients (2 male, 3 female) ranged in age from 31 to 82 years (mean, 56 y). The extent of disease varied from isolated involvement of the nasal cavity or the lacrimal gland to multicentric disease affecting the sinuses, nasal tract, and lower respiratory tract. The duration of symptoms ranged from 6 months to >20 years. The demographic features of the patients and disease extent were consistent with previously published reports of EAF, except for involvement of the lower respiratory tract in 1 case. Four of the 5 cases showed concentric perivascular fibrosis surrounding small-caliber vascular channels, embedded in an inflammatory infiltrate composed of lymphocytes, plasma cells, and eosinophils. One lacrimal gland biopsy showed a periductal inflammatory infiltrate, and 2 cases showed a storiform pattern of fibrosis. The index case had a serum IgG4 concentration of 1490 mg/dL (normal, 8 to 140 mg/dL). IgG4-positive plasma cells were identified in biopsies from 4 of the 5 cases. The numbers of IgG4-positive plasma cells ranged from 43 to 118 per high-power field, and the IgG4:IgG ratios ranged from 0.68 to 0.97. Neither IgG4-bearing nor IgG-bearing plasma cells were identified in 1 patient, whose longstanding disease was characterized principally by concentric perivascular fibrosis.'] Is eosinophilic angiocentric fibrosis a form of IgG4-related systemic disease?