Background: It is often challenging to differentiate between IgG4-related disease (IgG4-RD) and antineutrophil cytoplasmic antibody-associated vasculitis (AAV) due to their similar clinical presentations. Recently, growing evidence has suggested a strong connection between AAV and IgG4-RD.

Methods: A 60-year-old woman was transferred to our hospital with fever and kidney dysfunction. Abdominal computed tomography revealed widespread infiltrative lesions in both kidneys. Methods: Laboratory tests and subsequent renal biopsy confirmed both antineutrophil cytoplasmic antibody-associated vasculitis and IgG4-related disease. Methods: We initiated plasmapheresis, oral cyclophosphamide, and high-dose glucocorticoids for treatment. Despite this, the patient's condition worsened, requiring emergency hemodialysis.

Results: After 3 months of continued immunosuppressive treatment, renal function improved and hemodialysis was discontinued.

Conclusions: Our case showed an overlap of AAV and IgG4-RD, which might support the hypothesis of an overlap syndrome of AAV and IgG4-RD. Clinicians should have a high index of suspicion when diagnosing fever of unknown origin, with the possibility of overlapping AAV and IgG4-RD.