Antibodies to Yersinia enterocolitica serotype 03, 05, 06 and 09 were measured by the micro-agglutination method in 445 healthy subjects and patients with Grave's disease (n=70), Hashimoto's disease (n=45) and thyroid tumor (n=29). In contrast to previous reports, the incidence of antibodies to serotype 03 in each group of patients with thyroid diseases was not significantly different from that in healthy subjects. However, the incidence of antibodies to serotype 05 was significantly higher in patients with Graves' disease (81.4%, P<0.001) and Hashimoto's disease (91.1%, P<0.001) than in healthy subjects (58.9%). Significantly increased incidence of antibodies to serotypes 06 and 09 was observed only in patients with Hashimoto's disease (40.0% and 51.1% vs healthy subjects 24.7% and 29.9%, respectively). Patients with thyroid tumor showed no increase in any serotype of Yersinia enterocolitica. No correlations was found between the titers of anti- Yersinia antibodies and antithyroglobulin or anti-microsomal antibodies. These data indicate an association between thyroid autoimmunity and antibodies to Yersinia enterocolitica. These results are different from those in reports from other countries, seggesting that serotype specificity may be influenced by racial or genetic factors.