Abstract. Two blood samples were taken at an interval of 5 years in a continuing epidemiological study of a rural population in south-western Finland with an age range of 40–64 years at commencement. Paired sera of 680 subjects were tested for antibodies against thyroglobulin and thyroid microsomal antigen. In the seroconversion cases the titres in the second specimens were either low or medium but never high. In the cases selected on the basis of the highest titres in the follow-up specimens, the titres in the baseline specimens were either high or medium but never low. These findings suggest that the emergence and the development of thyroid autoimmune seroreactions are slow processes.