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Immunoglobulin G4‐related disease (IgG4‐RD)‐associated hypercalcemia has rarely been reported. We report a case of IgG4‐RD that presented as severe symptomatic hypercalcemia. A 50‐year‐old woman with a history of sustained bilateral periorbital swelling and proptosis for more than 5 years presented to our hospital complaining of a 3‐day history of significant and progressive nausea, vomiting, loss of appetite, fatigue, and pruritus. She denied a long history of medication. On admission, laboratory tests showed severe hypercalcemia with serum adjusted calcium elevated to 4.34 mmol/L and renal dysfunction with serum creatinine elevated to 206 μmol/L. Urinary calcium excretion was increased. The serum IgG4 subclass was markedly elevated to 22.4 g/L with polyclonal hypergammaglobulinemia. Tests of autoantibodies were all negative. Bone metabolism markers that reflect the activity of osteoblasts and osteoclasts were all significantly elevated. However, the levels of intact parathyroid hormone and 25(OH) vitamin D3 were decreased. B‐ultrasonography showed chronic inflammation of bilateral submandibular glands. Neither bone marrow biopsy nor positron emission tomography – computed tomography examination showed evidence of neoplastic diseases. The patient was treated with intravenous saline infusion, loop diuretics, salmon calcitonin, glucocorticoids, and hemodialysis with a good response. [ABSTRACT FROM AUTHOR]