The wide racial-geographic differences in the incidence and prevalence of insulin-dependent diabetes mellitus (IDDM) between Europids and Asian populations prompted us to compare frequencies of positivity of autoantibody to glutamic acid decarboxylase (GAD). The patients with IDDM included 41 Koreans, 30 Thais, and 45 Australian Europids; the Koreans included 14 cases regarded as atypical IDDM by reason of a delayed requirement for insulin treatment. Autoantibodies were measured by radioimmunoprecipitation using iodinated purified porcine brain GAD. The frequency of positive tests for anti-GAD of 30% (8/27) for Koreans and 51% (20/39) for Thais was significantly lower than the 84% (38/45) for Australian Europids, even after stratifying by age of onset. Correspondingly, the mean levels of anti-GAD among seropositive cases were significantly lower for Koreans than for Australian Europids. In contrast to Thais and Australians, more than half the Koreans were diagnosed at age > 20 years, but there was no significant difference in positivity for anti-GAD between those over or under the age of 20 at diagnosis. The different frequency of positivity in tests for anti-GAD among Koreans, Thais, and Australian Europids with IDDM suggests that there is a greater etiologic heterogeneity of IDDM among Asian than Europid populations, in whom autoimmune destruction of pancreatic islets predominates.