BACKGROUND: Type 1 diabetes mellitus is frequently associated with other autoimmune diseases. The broad concept of polyendocrinopathies takes into consideration that patients affected by at least one endocrine disease may have another autoimmune disorder or express specific autoantibodies. Anti-glutamic acid decarboxylase autoantibodies, now recognized as one of the major serological markers for type 1 diabetes has been reported to be higher in type 1 diabetes patients with autoimmune thyroid diseases (ATD) than in those without ATD. The objective of the present study was to evaluate the prevalences of GAD65 antibodies applying a newly developed assay(anti-GAD65) in type 1 diabetes patients with and without ATD. METHODS: We developed a new anti-GAD65 assay after mammalian expression of a recombinant GAD65 antigen. Since the detection of anti-GAD65 is rather complicated and insensitive due to inherent antigenic difference of antibody recognition in conventional assays, we applied this new approach in measuring anti-GAD autoantibodies and compared the result with ICA and anti-GAD measurement using the purified porcine GAD (anti-GAD) in 109 cases of type 1 diabetes, 29 of whom had concomitant ATD (mean age at diagnosis: 7.9 yr, mean duration of type 1 diabetes: 4.5 yrs). RESULTS: The overall prevalence of anti-GAD65 antibodies was 65% (71 of 109) in patients with Korean type 1 diabetes. Prevalences and titers of anti-GAD65 had not changed much after controlling for the duration and the status of concomitant ATD. In contrast, the prevalence of anti-GAD was 56%(61 of 109), while that of ICA(+) WAS 36% in type 1 diabetes patients. We found significant, but not strong association of anti-GAD65 either with anti-GAD(r=0.4, p<0.01) or with ICA(r=0.6, p< 0.001). CONCLUSION: From this, we could assess that autoantibodies are present at comparable sensitivity and specificity in Korean type 1 diabetes patients. This anti-GAD65 assay, another immunologic marker for type 1 diabetes might also confer disease susceptibility among Koreans, but no increase in the prevalence or in the titer in patients with ATD may suggest that this marker is unlikely to give much benefit, for the detection of the overlapping disease of type 1 diabetes and ATD.