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The association between IgG4 and inflammatory pseudotumor was first reported with regard to pancreatic pseudotumor, which is well known as sclerosing pancreatitis or autoimmune pancreatitis. In addition, there is increasing evidence that IgG4 is also involved in inflammatory pseudotumor of the extrapancreatic tissue. In this report, we present a case of IgG4-related inflammatory pseudotumor of the breast. A 46-year-old woman presented with an induration in the left breast. Radiologic examination revealed an ill-circumscribed mass measuring 1.6 cm in diameter in the left breast. Breast cancer could not be ruled out radiologically, and excision biopsy was performed for a definite diagnosis. Histologically, this nodule was composed of an irregular proliferation of stromal cells associated with severe lymphoplasmacytic infiltration, obliterative phlebitis, and eosinophils. No atypical feature regarding the stromal cells or lymphocytes was observed. Furthermore, many plasma cells within the lesion were immunohistochemically positive for IgG4, and the serum IgG4 concentration of this patient was elevated on postoperative examination. This case suggests that inflammatory pseudotumor of the breast has a pathogenetic process similar to that of sclerosing pancreatitis. IgG4 might become a useful marker for inflammatory pseudotumor of the breast, and it might benefit from steroid therapy, as does sclerosing pancreatitis.