Fas and TNF receptor I (TNF-RI) share homology at their cytoplasmic death domain and belong to the same gene family, but utilize different pathways to signal activation-induced cell death. To determine the combined effects of defective TNF-RI and Fas signaling on lymphadenopathy and autoimmune disease, we backcrossed TNF-RI knockout mice (Tnfr1%) with Fasdeficient C57BL/6-lpr/lpr mice. Tnfr1%lpr/lpr mice developed greatly accelerated lymphadenopathy and autoantibody production compared with C57BL/6-/pr/ lpr mice. Tnfr1%-lpr/lpr mice also exhibited high mortality and early onset autoimmune disease characterized by massive inononuclear cell infiltration in liver, kidney, lung, and knee joints. These results indicate that the Fas-mediated apoptosis defect in lpr mice is accelerated in the absence of TNF-RI and that normal expression of TNF-RI might partially compensate for the Fas-mediated apoptosis defect of mononuclear cells in lpr mice.