Type 1 diabetes is a heterogeneous autoimmune disease and is frequently associated with other organ-specific autoimmune diseases, including autoimmune thyroid disease (AITD). Type 1 diabetic patients with AITD are known to show distinct clinical and immunological features from patients without AITD. This study investigated whether interleukin-10 (IL-10) gene promoter region polymorphisms are associated with susceptibility to type 1 diabetes and AITD. The frequency of -1082G/A, -819C/T, and -592C/A polymorphisms was analyzed in 54 type 1 diabetic patients with AITD, 74 type 1 diabetic patients without AITD, 124 nondiabetic patients with AITD, and 107 healthy subjects in a case-control study. No significant differences on the allele and genotype frequencies of three polymorphisms were found not only in type 1 diabetic patients with AITD compared with normal controls, but also between nondiabetic patients with AITD and healthy controls. The distribution of IL-10 gene haplotypes was also similar between both patient groups and normal controls. These results suggest that IL-10 gene promoter region polymorphisms are not associated with genetic susceptibility to type 1 diabetes and AITD.