Since Hamano et al. have first reported serum IgG4 elevation in sclerosing pancreatitis in 2001, various systemic disorders have been reported to be related to elavated IgG4, and many names have been proposed from the point of view of the systemic condition. Despite similarities in the organs damaged in IgG4-related Mikulicz's disease and Sjögren's syndrome, there are marked clinical and pathological differences between the two entities. IgG4-related Mikulicz's disease and Küttner's tumor are related diseases and complete differentiation is very difficult. The majority of cases diagnosed with autoimmune pancreatitis in Japan are IgG4-related sclerosing pancreatitis, and it should be recognized that this is distinct from the western type. There is a likelihood that cases once diagnose as Castleman's disease that showed good responsiveness to glucocorticoid treatment may have been IgG4-related lymphadenopathy, and should be re-assessed in light of recent findings. Diagnosis of IgG4-related disease is defined by both 1) Elevated serum IgG4 (>135 mg/dl) and 2) Histopathological features including lymphocyte and IgG4(+) plasma cell infiltration (IgG4(+) plasma cells/IgG(+) plasma cells >50% on a highly-magnified slide checked in five points), however differential diagnosis from other distinct disorders, such as sarcoidosis, Castleman's disease, Wegener's granulomatosis, lymphoma, cancer, and other existing conditions is necessary. To avoid diagnostic confusion, simpler and more scientific names should be used where disease-specific pathogenesis or markers have been ascertained.