Fas (CD95) is a cell surface death receptor belonging to the tumor necrosis factor receptor superfamily, which mediates apoptosis-inducing signaling when activated by Fas ligand or its agonistic antibody. lpr mice with a loss of apoptosis-inducing function mutation in the Fas gene develop systemic autoimmune disease and lymphadenopathy but not allergic inflammation. In the case of Fas mutations including lpr and knockout (KO), background genes determine the incidence and severity of lymphadenopathy and histopathological manifestation of systemic autoimmunity: MRL-lpr/lpr mice and C57BL/6-lpr/lpr or C57BL/6 Fas KO mice develop severe and minimum disease, respectively. We generated Fas KO mice with the Balb/c background that show severer autoimmune phenotypes than MRL-lpr/lpr mice, such as critical infiltration of mononuclear cells into lung, liver and spleen, elevated serum levels of auto-antibodies and a decreased life span. To our astonishment, Balb/c Fas KO mice spontaneously develop blepharitis with not only autoimmune inflammation with deposition of auto-antibody but also allergic inflammation with infiltration by eosinophils and mast cells and show the capacity to strongly increase serum level of IgE and IgG1 along with their aging. Thus, Fas expression regulates development of not only autoimmune disease but also allergic inflammation.