Objectives. In order to clarify the relationship between enteroviruses and type 1 diabetes mellitus in Japan we investigated enteroviral RNA in serum from children with type 1 diabetes mellitus.Methods. We investigated enteroviral RNA in serum from children with type 1 diabetes mellitus by using highly sensitive RT-PCR. Additionally the sequences and viral loads were determined and compared with anti-coxsackie virus antibodies and anti-glutamic acid decarboxylase (GAD) antibodies.Results. RT-PCR for enterovirus was positive in 23 (37.7%) from 61 samples. The positivity had no disparity of age, but decreased by aging after the occurrence of type 1 diabetes mellitus. The sequences of the positives were similar as those of coxsackie B4. The viral loads revealed that there was no positive patient with high titers of anti-GAD antibodies.Conclusion. In Japan there is some correlation with type 1 diabetes mellitus and enterovirus. The pathophysiology of type 1 diabetes mellitus seems to consist of a direct destruction by persistent coxsackie virus and the autoimmune mechanism through autoantibodies against beta-cells.