During 1971 through 1978 antibodies against Yersinia enterocolitica types 3 and 9 and Yersinia pseudotuberculosis IA were measured in 25307 patients with symptoms compatible with yersinia infections in southwestern Finland. Agglutinin titres greater than or equal to 160 were found in 1 500 patients. The annual incidence of elevated titres varied, but no significant overall change was found during this period. The results suggest that yersiniosis is not a "new" infection with increasing incidence, but an endemic infection in this region.