Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) is a monogenic autosomal recessive disease caused by mutations in the AIRE gene. Here we have produced knock-out mice for the Aire gene. The Aire^–/– mice develop normally; however, autoimmune features of APECED in Aire^–/– mice are evident, including multiorgan lymphocytic infiltration, circulating autoantibodies and infertility. The distribution of B and T cells and thymic maturation as well as activation of T cells appear normal, while the TCR-Vβ repertoire is altered in peripheral T cells of Aire^–/– mice. When mice are challenged with immunization, the peripheral T cells of Aire^–/– mice have a 3–5-fold increased proliferation. These findings suggest that the Aire gene is not necessary for normal T cell education and development, while a defect in immune response detected in challenged Aire^–/– mice underlines the crucial role of AIRE/Aire in maintaining homeostatic regulation in the immune system.