The sera for 88 parents and 9 siblings of 73 patients with insulin dependent diabetes mellitus in childhood and 437 controls matched in age and sex, were tested by the thyroglobulin and microsome-coated tanned red cell hemagglutination test (Fuji-Zoki Co. Tokyo). None of 73 children with diabetes mellitus had antithyroglobulin antibodies, whereas twelve (16.4%) had antimicrosomal antibodies compared with the incidence of 0.4% and 1.1%, respectively, in 437 controls. In the parents and siblings of these probands, thyroid antibodies were also found in increased incidence. The incidence of antimicrosomal antibodies in the 68 mothers was significantly higher than in controls matched for age and sex, but the incidence of the positive thyroid antibodies in the 20 fathers and 9 siblings was not significantly different from that in control populations. The incidence of thyroid antibodies tended to be higher, though not significant, in parents and siblings of diabetic children with positive thyroid antibodies than in those of diabetics with negative ones. These findings suggest that immunogenetic factors may be responsible for the pathogenesis of some cases of diabetes mellitus in childhood.