CD4+ T lymphocytes are required to induce spontaneous autoimmune diabetes in the NOD (Non Obese Diabetic) mouse. Since pancreatic β cells upregulate Fas expression upon exposure to proinflammatory cytokines, we studied whether the diabetogenic action of CD4+ T lymphocytes depends on Fas expression on target cells. We assayed the diabetogenic capacity of NOD spleen CD4+ T lymphocytes when adoptively transferred into a NOD mouse model combining: a) Fas-deficiency, b) FasL-deficiency, and c) the SCID mutation. We found that CD4+ T lymphocytes require Fas expression in the recipients’ target cells to induce diabetes. IL-1β has been described as a key cytokine involved in Fas up-regulation on mouse β cells. We addressed whether CD4+ T cells require IL-1β to induce diabetes. We also studied spontaneous diabetes onset in NOD/ICE (Interleukin-1 Converting Enzyme) deficient mice, in NOD/IL-1β deficient mice, and CD4+ T cell-adoptively transferred diabetes into NOD/SCID IL-1β-deficient mice. Neither IL-1β nor IL-18 are required for either spontaneous or CD4+ T-cell adoptively transferred diabetes. We conclude that CD4+ T cell-mediated β cell damage in autoimmune diabetes depends on Fas expression, but not on IL-1β, unveiling the existing redundancy regarding the cytokines involved in Fas upregulation on NOD β cells in vivo.