Type 1 diabetes (T1D) is frequently associated with other autoimmune diseases. The occurrence of common features of autoimmune diseases and the coassociation of multiple autoimmune diseases in the same individual or family support the notion that there may be common genetic factors. We previously reported that immunogenetic markers of T1D were similar in Korean and U.S. families. We hypothesized that nonislet autoimmunity might be expressed in a similar manner in Korean as well as in Caucasian patients and correlated with the DRB1-DQB1 haplotypes. Twenty-five percent of Korean patients with T1D (32/128) had antithyroid autoantibodies (ATA+), whereas eight percent of age-matched controls were ATA+ [OR = 3.7 (95% CI:1.0-12.0), P < 0.05]. Twenty-three percent (84/369) of age-matched T1D patients from the U.S. had ATA (not statistically different from that in Koreans). None of the Korean patients nor the family members were positive for 21-hydroxylase (21OH) or transglutaminase autoantibodies (TG). In contrast, in U.S. patients, nine T1D patients (2.4%) were 21OH+ (not statistically different from that in Koreans) and 39 T1D patients (10.6%) were TG+ [OR = 30.7 (95% CI: 4.6-111), P < 10(-4) vs. Korean patients]. Nonislet autoimmunity is prevalent in Korean and U.S. patients with T1D and their family members, but specific forms of autoimmunity differ even after matching for their HLA haplotypes. Individuals with T1D and their relatives frequently develop a panel of autoantibodies, perhaps due to other common susceptibility genes that are shared among first-degree relatives.