It has been established that insulin-dependent diabetes mellitus (IDDM) in nonobese diabetic (NOD) mice results from a CD4⁺ and CD8⁺ T cell–dependent autoimmune process directed against the pancreatic beta cells. The precise roles that beta cell–reactive CD8⁺ and CD4⁺ T cells play in the disease process, however, remain ill defined. Here we have investigated whether naive beta cell–specific CD8⁺ and CD4⁺ T cells can spontaneously accumulate in pancreatic islets, differentiate into effector cells, and destroy beta cells in the absence of other T cell specificities. This was done by introducing K^d– or I-A^g7–restricted beta cell–specific T cell receptor (TCR) transgenes that are highly diabetogenic in NOD mice (8.3- and 4.1-TCR, respectively), into recombination-activating gene (RAG)-2–deficient NOD mice, which cannot rearrange endogenous TCR genes and thus bear monoclonal TCR repertoires. We show that while RAG-2^−/− 4.1-NOD mice, which only bear beta cell–specific CD4⁺ T cells, develop diabetes as early and as frequently as RAG-2⁺ 4.1-NOD mice, RAG-2^−/− 8.3-NOD mice, which only bear beta cell–specific CD8⁺ T cells, develop diabetes less frequently and significantly later than RAG-2⁺ 8.3-NOD mice. The monoclonal CD8⁺ T cells of RAG-2^−/− 8.3-NOD mice mature properly, proliferate vigorously in response to antigenic stimulation in vitro, and can differentiate into beta cell–cytotoxic T cells in vivo, but do not efficiently accumulate in islets in the absence of a CD4⁺ T cell–derived signal, which can be provided by splenic CD4⁺ T cells from nontransgenic NOD mice. These results demonstrate that naive beta cell– specific CD8⁺ and CD4⁺ T cells can trigger diabetes in the absence of other T or B cell specificities, but suggest that efficient recruitment of naive diabetogenic beta cell–reactive CD8⁺ T cells to islets requires the assistance of beta cell–reactive CD4⁺ T cells.