IgM, IgG and IgA class serum antibodies against Yersinia enterocolitica O:3 and O:9 and Yersinia pseudotuberculosis 1a and 3 in 41 patients with thyroid disease and 50 healthy control persons were measured by enzyme-linked immunosorbent assay (ELISA). Concentrations of antibody levels against Yersinia enterocolitica O:9 and Yercinia pseudotuberculosis 1a and 3 did not differ significantly between the patients and controls. The median value of IgA class antibody Yersinia enterocolitica O:3 was 7.5 relative units (EIU, percentage of the reference serum) in the patients with thyroid disease and 0.7 EIU in the controls (P less than 0.01; Mann-Whitney's U-test), whereas IgM and IgG class antibodies did not show this difference. IgA class serum antibodies were especially high in patients with autoimmune thyroid diseases. These findings and two case reports support the concept that there may be a causal relationship between infections with Yersinia enterocolitica O:3 and autoimmune thyroid diseases.