Immunoglobulin G4 (IgG4)-related disease is characterized by elevated serum IgG4 levels and increased numbers of IgG4-positive cells. However, its pathogenesis is not fully understood. We previously suggested that mast cells may play an important role in IgG4-related disease. In this study, we confirmed the characteristics of mast cells in IgG4-related lymphadenopathy by using immunohistochemistry and dual immunofluorescence. We analyzed 23 cases of IgG4-related lymphadenopathy and compared them with 23 cases of non-specific lymphoid hyperplasia. The majority of patients with IgG4-related lymphadenopathy had cervical lesions with involvement of other organs. Immunohistologically, mast cells with strong cytoplasmic staining for immunoglobulin E and high affinity immunoglobulin E receptor were significantly increased in IgG4-related lymphadenopathy as compared to those in non-specific lymphoid hyperplasia (mean: 3.83 ± 3.99 cells per high power field and 7.14 ± 8.21 cells per high power field, respectively; P = 0.007 and P = 0.011). In addition, dual immunofluorescence assay showed that immunoglobulin E and high affinity immunoglobulin E receptor staining exhibited a cytoplasmic granular pattern in IgG4-related lymphadenopathy, suggesting internalization of the antibodies and receptors. Our findings showed that mast cell activation might be involved in the pathogenesis of IgG4-related disease.