Reprints and permission:

No sponsorships or competing interests have been disclosed for this article. Objective. IgG4-related disease (IgG4RD) causing sinonasal and skull base pathology is uncommonly described. We pres-ent a series of suspected IgG4RD patients, with a pertinent review of the literature to highlight diagnostic challenges. Study Design. Case series. Setting. Academic tertiary care center. Subjects and Methods. Case series of patients with IgG4RD or suspected IgG4RD involving the sinonasal cavity and skull base. Results. We present 4 patients with atypical sinonasal and/or skull base disease who were noted to have IgG4-positive plasma cell infiltration on immunohistochemistry of biopsy specimens. IgG4RD, a recently described entity affecting mul-tiple organs, is characterized by lymphoplasmacytic infiltration and often elevated serum IgG4. IgG4RD can masquerade as malignancy or infection but responds to glucocorticosteroid and immunosuppressant therapy. IgG4RD has been infre-quently reported presenting as sinonasal or skull base lesions, and definitive diagnostic criteria for these regions are not established. In our series, IgG4RD was suspected in all 4 patients, but only 1 met all current criteria for definitive diag-nosis. All 4 patients, however, responded to corticosteroid therapy, and 1 was placed on long-term azathioprine. Conclusion. IgG4RD is rarely described in the sinonasal cavity and skull base, and specific diagnostic criteria for such disease have not been defined. We present a series of patients with IgG4-positive plasma cell inflammatory pathology who were suspected to have IgG4RD. Our series highlights diagnostic challenges associated with these patients. Tumefactive and destructive sinonasal-skull base lesions with a plasma cell-rich infiltrate should incite suspicion of IgG4RD, and immunohisto-chemistry for IgG4-positive plasma cells should be performed