The insulin autoimmune syndrome (IAS), or Hirata's disease, is characterized by the combination of fasting hypoglycemia, high concentration of total serum immunoreactive insulin, and presence of autoantibodies to native human insulin in serum. Autoantibody production is classified as monoclonal or polyclonal, with the majority of IAS cases classified as polyclonal. Previously, we observed a striking association between the human leukocyte antigen (HLA) class II alleles DRB1*0406/DQA1* 0301/DQB1*0302 and Japanese IAS patients with polyclonal insulin autoantibodies (IAAs) and T-cell recognition of human insulin in the context of DRB1*0406 molecules. Because of such a strong HLA association in IAS, we performed intra- and interethnic studies on IAS-associated DRB1 alleles and searched for the critical amino acid residue(s) for IAS pathogenesis. Glutamate at position 74 in the HLA-DR4 beta 1-chain was presumed to be essential to the production of polyclonal IAA in IAS, whereas alanine at the same position of the HLA-DR beta 1-chain might be important in the production of monoclonal IAA.