Here, we report a case of allogeneic islet transplantation in Japan. A 48-year-old man received intraportal islet transplantation (5,945 islet equivalent/kg), and stabilization of blood glucose levels and suppression of hypoglycemia were achieved. In the present case, we used our original assessment method to detect the responses of the recipient's T cells to islet autoantigens over time to monitor cellular autoimmunity. Other markers could not predict graft dysfunction in advance, but our method detected the activation of islet antigen-specific CD8(+) T-cell responses before the deterioration of pancreatic beta-cell function, indicating the possibility of the non-invasive detection of pancreatic beta-cell damage due to recurrent autoimmunity.