Insulin-dependent diabetes mellitus (IDDM) develops mainly from the destruction of pancreatic beta cells by cytotoxic T lymphocytes. One of the key phenomena observed in the pancreas of IDDM patients is the destructive process of beta cells by cytotoxic T cells. In NOD mice, an animal model of IDDM, the T cell receptor (TCR) of the infiltrating T lymphocytes was reported to be variable. Our analysis of RNAs obtained from the pancreas of newly diagnosed IDDM patients using the RT-PCR method indicated that the repertoire of T cell receptor alpha was restricted. Clustering of conservative amino acid sequences was found in each patient. The elucidation of the TCR of T cells recognizing and destructing beta cells in IDDM would enable us to develop a novel method immunologically intervening the occurrence of IDDM.