antibodies, gastric parietal cell antibodies (PCA) and antinuclear factors were studied in208insulin dependent diabetic (IDD) according to the duration of diabetes and patient's age at the time of testing. Antimicrosomal antibodies were found in11out of47 (23.4%) IDD with the duration of less than one year, however this value declined to13.1% at1to3years, 15.3% at4to5 years, 10.8% at6to10yearsand5.8% at more than10years. Of the47IDD,7 (14.8%) were positive for gastric parietal cell antibodies.The prevalence of PCA declined with the increasing duration of diabetes.However, this decrease in the prevalence of antimicrosomal antibodies and PCA was not so extreme as that of pancreatic islet cell antibodies.Antinuclear factors did not reveal a significant correlation with the duration of diabetes.In normal controls, the prevalence of antimicrosomal antibodies, PCA and the antinuclear factors increased progressively with age. In IDD, the prevalence of the antinuclear factors was also progressively greater with age. However, the prevalence of antimicrosomal antibodies in IDD decreased with age and those of PCA showed the lowest percent in the40-69year-age group.There is an increasing evidence that autoimmune mechanism may have a role in the pathogenesis in insulin dependent diabetics (IDD). The clinical association between IDD and autoimmune diseases and a high incidence of thyroid or gastric autoantibodies and an antipancreatic cellular hypersensitivity in IDD have already been reported (Hecht et al., 1968;Irvine et al., 1970;Nagaoka et al., 1979; Nerup et al., 1971 Nerup et al., , 1973. By mean of the indirect immunofluorescence test, Bottazzo et al.(1974) and MacCuish et al.(1974) found respectively pancreatic islets cell antibodies (I. C. Ab.) in IDD with autoimmune polyendocrine disease. Using an improved technique and group-0 fresh human pancreas, Irvine et al.(1977) determined the prevalence of I. C. Ab. in IDD with and without other organ-specific autoimmune disorders and they found that I. C. Ab. declined with the duration. In collaboration with W. J. Irvine, the inverse relationship between the prevalence of I. C. Ab. and the duration of diabetes was already observed by us in Japanese insulin dependent diabetics (Nagaoka et al., 1978). The purpose of the present study was to study the relationship between thyroid antibodies, gastric parietal cell antibodies (PCA) and the duration of diabetes.Received January26, 1979.