Interleukin (IL)-12 is a key factor in cell-mediated immunity that drives the development of Th1 cells and stimulates T lymphocytes and natural killer cells to produce interferon (INF)-γ. The IL-12B gene, which encodes the p40 subunit of IL-12, is located at chromosome 5q31-33 and a linkage finding for autoimmune thyroid disease (AITD) on 5q31-33 in a Japanese population has been reported. It is also reported that the A/C polymorphism in the 3' untranslated region (UTR) of the IL-12B gene (1188A/C) is associated with IL12B mRNA expression levels. We attempted to determine whether genetic polymorphisms of the IL-12B gene are associated with AITD. One hundred three patients with Hashimoto's thyroiditis, 90 patients with Graves' disease, and 123 healthy control subjects were recruited. We detected the 1188A/C polymorphism using a PCR-RFLP method and the A/T polymorphism in intron 4 of the IL-12B gene using a cycle sequencing method. These IL-12B gene polymorphisms showed strong linkage disequilibrium, and their genotype and allele frequencies in the patients did not differ from those in the control subjects. Our results suggest that IL-12B gene polymorphisms were unlikely to have an effect on the development of Hashimoto's thyroiditis or Graves' disease in Japanese patients.