IgG4-related systemic disease/systemic IgG4-related disease has been established as a new systemic disease entity. It is characterized by high serum IgG4 concentrations and abundant IgG4-bearing plasma cell infiltration in the involved organs. The chronic inflammation can attack lacrimal glands, salivary glands, the thyroid, lung, pancreas, kidney, and prostate. The concept includes Mikulicz's disease, Riedel's thyroiditis, pulmonary fibrosis, pulmonary pseudotumor, autoimmune pancreatitis, a part of tubulointerstitial nephritis, and chronic prostatitis. It is important to note that these lesions can occur at different times and sites. So, it is necessary to reconfirm the disease definition and entity in each specialized field. The diagnosis of this disease is confirmed by the above serological and histopathological characteristics. There are clinical diagnostic criteria of Mikulicz's disease (the Japanese Medical Society for Sjögren's Syndrome) and autoimmune pancreatitis (the Japanese Ministry of Health, Labour and Welfare, and the Japan Pancreas Society). They are convenient and useful. Glucocorticoid improves the physical abnormalities, and the initial dose of prednisolone is 30 mg/day, tapered in 5-mg reductions every two weeks. Nevertheless, there are some cases unable to achieve complete remission.