Particular major histocompatibility complex (MHC) class II alleles clearly contribute to T cell-mediated autoimmune type 1 diabetes (T1D) in both humans and nonobese diabetic (NOD) mice. However, studies in NOD mice indicate MHC class I-restricted T cell responses are also essential to T1D development. In humans, epidemiological studies have suggested that some common class I alleles, including HLA-A2.1 (A*02011), may confer increased susceptibility to T1D when expressed in conjunction with certain class II alleles. We show here that when HLA-A2.1 molecules are transgenically expressed in NOD mice, A2-restricted T cell responses arise against pancreatic β cells, leading to an earlier onset of T1D. The accelerated onset of T1D in the NOD.HLA-A2.1 transgenic mice is not due to nonspecific effects of expressing a third class I molecule, because a stock of NOD mice transgenically expressing HLA-B27 class I molecules showed no such acceleration of T1D, but rather were significantly protected from disease. These findings provide the first functional evidence that certain human MHC class I molecules can contribute to the development of T1D.