Genetic factors clearly contribute to the development of autoimmune destruction of pancreatic beta cell in IDDM. The HLA class II genes have an important role in the pathogenesis of Japanese IDDM as they do in Caucasians. Particular HLA class II alleles and haplotypes contribute to the IDDM susceptibility/resistance in Japanese. In the 5' insulin gene polymorphism, the shorter insertion (class 1 allele) is predominant in Japanese, and thus is not associated with IDDM. Subdivision of the class 1 alleles also failed to show an association with Japanese IDDM. The insulin gene region appears not to be a valuable genetic marker for IDDM in Japanese. Recently, we found an association of polymorphism in the IFN-gamma gene with Japanese IDDM.