Elevated IgG4 is characteristic of cases of IgG4-RD, a newly recognized systemic disease. However, several chronic inflammatory conditions, including rheumatic diseases, can also be associated with increased levels of IgG4. There have also recently been several reports describing an increased IgG4 immune response to some vasculitis syndromes, in particular Churg-Strauss syndrome and granulomatosis with polyangiitis. To avoid misdiagnosis, clinicians must be aware that the clinical manifestations of IgG4-RD and ANCA-associated vasculitis may overlap. The meaning of these observations is not yet understood, and more studies are needed to determine the true significance of the increased IgG4 response to vasculitis syndromes, especially anti-neutrophil cytoplasmatic antibodies (ANCA)-associated vasculitis.