IgG4-related diseases are characterised by tissue infiltration of IgG4-positive plasma cells and diffuse or localised fibrosis. Elevated serum IgG4 is also present. With time the subsequent organs are affected; thus high levels of IgG and IgG4 should prone to systematic control to prevent multiorgan dysfunction. Detailed diagnostics is necessary and should include radiologic, ultrasonographic and tomographic evaluation, measurements of serum IgG4 and laboratory parameters specific to dysfunction of the affected organ. An isolated infiltration by IgG4+ plasmocytes or elevated IgG4/IgG ratio constitutes relatively non-specific findings. Thus, during diagnostics the combined presence of the characteristic histopathological appearance, increased numbers of IgG4+ plasma cells, elevated serum IgG4 and organomegaly with progressing organ dysfunction is necessary. Glucocorticosteroids are effective in most cases (at least at the beginning of treatment), but disease flares are common. The effectiveness of biologic therapy is also underlined in the literature.