Nonobese diabetic (NOD) mice transgenic for Fas ligand (FasL) on islet β cells (HIPFasL mice) exhibit an accelerated diabetes distinct from the normal autoimmune diabetes of NOD mice. This study was undertaken to define the mechanism underlying accelerated diabetes development in HIPFasL mice. It was found that diabetes in HIPFasL mice is dependent on the NOD genetic background, as HIPFasL does not cause diabetes when crossed into other mice strains and is lymphocyte dependent, as it does not develop in HIPFasLSCID mice. Diabetes development in NODSCID recipients of diabetic HIPFasL splenocytes is slower than when using splenocytes from diabetic NOD mice. β cells from HIPFasL mice are more susceptible to cytokine-induced apoptosis than wild-type NOD β cells, and this can be blocked with anti-FasL Ab. HIPFasL islets are more rapidly destroyed than wild-type islets when transplanted into nondiabetic NOD mice. This confirms that FasL+ islets do not obtain immune privilege, and instead NOD β cells constitutively expressing FasL are more susceptible to apoptosis induced by Fas-FasL interaction. These findings are consistent with the accelerated diabetes of young HIPFasL mice being a different disease process from the autoimmune diabetes of wild-type NOD mice. The data support a mechanism by which cytokines produced by the insulitis lesion mediate up-regulation of β cell Fas expression, resulting in suicide or fratricide of HIPFasL β cells that overexpress FasL.