Online Articles: Case ReportsLarge Tumefactive IgG4-related Disease Histologic, Cytologic, and Immunohistochemical Features of a Very Unusual CaseAkhtar, Israh MD*; Shenoy, Veena MD*; Khan, Majid MD†; Saad, Ali G. MD*Author Information Departments of *Pathology †Radiology, University of Mississippi Medical Center, Jackson, MS The authors declare no conflict of interest. Reprints: Ali G. Saad, MD, Department of Pathology, University of Mississippi Medical Center, 2500 N. State Street, Jackson, MS 39216 (e-mail: ASAAD@UMC.EDU). Received June 19, 2017 Accepted June 29, 2017 Online date: March 11, 2020 Applied Immunohistochemistry & Molecular Morphology: March 2020 - Volume 28 - Issue 3 - p e21-e25 doi: 10.1097/PAI.0000000000000571 Buy Metrics Abstract Immunoglobulin G4-related disease (IgG4-RD) is a regional or systemic multiorgan lymphoplasmacytic inflammatory disease of unknown etiology. It has been described in numerous organs and anatomic locations. Review of the literature shows that when the disease involves the retroperitoneum it causes retroperitoneal fibrosis. Tumefactive IgG4-RD of the retroperitoneum has not been previously reported. In this report, we describe the first case of a large retroperitoneal tumefactive IgG4-RD along with its histologic, cytologic, and immunohistochemical characteristics. Copyright 2020 Wolters Kluwer Health, Inc. All rights reserved.