The frequency of agglutinating antibodies to Yersinia enterocolitica (Y. ent.) serotype 3 and 9 has been studied in sera from patients with various thyroid diseases. No antibodies to Y. ent. serotype 9 were found. Titres (≧ 10) of Y. ent. serotype 3 antibodies were found significantly more frequently in thyroid patients than in controls. In patients with Graves' disease the frequency was 59.5 per cent compared with 16.6 and 27.7 % in the two control groups (P < 0.0005). High titres (≧ 80) were significantly increased in patients with Graves' disease and diffuse nontoxic goitre when compared with the control groups. The prevalence of Y. ent. antibodies decreased with the duration of the disease. In patients with recurrent Graves' disease high titres of Y. ent. antibodies were found in 25 per cent compared with 11 per cent in patients with the first attack of Graves' disease (P < 0.05). No correlation with age, sex, exophthalmos or the presence of antibodies to thyroid antigens was found. As the O antigen of Y. ent. serotype 3 is specific the detection of these antibodies suggest an infection with Y. ent. and possibly an association between this infection and thyroid disorders, in particular Graves' disease.