Aims The cutaneous manifestation of IgG₄-related disease has rarely been reported. The aim of this study is to identify and describe the cutaneous manifestations associated with IgG₄-positive plasma cell infiltration in the skin.

Methods The authors investigated two cases of IgG₄-related disease with solitary skin lesions and compared the immunohistochemical characteristics of infiltrating cells among IgG₄-related disease, Kimura's disease and cutaneous pseudolymphoma.

Results IgG₄-related disease manifested as an indurated plaque on the anterior chest in one case and a nodule on the toe in the other case. Histopathologically, skin lesions of IgG₄-related disease showed a dense, mixed-cell infiltrate containing lymphocytes, plasma cells and eosinophils along with fibrosis. Plasma cells stained positively with anti-IgG and anti-IgG₄ antibodies, and the ratio of IgG₄+/IgG+ cells was more than 50%. Serum levels of IgG and IgG₄ were not elevated and no lesions were found in other organs. Skin samples taken from Kimura's disease showed histopathological features similar to those of IgG₄-related disease. The proportion of IgG₄+/IgG+ was high in Kimura's disease, but not in cutaneous pseudolymphoma.

Conclusions The solitary skin lesions of IgG₄-related disease were similar histologically and immunohistochemically to the skin lesions of Kimura's disease. The concept of IgG₄-related disease may help clarify the pathomechanism of diseases of unknown aetiology that possess features of IgG₄-related disease.