Primary Sjögren's syndrome (SS) is an autoimmune disorder of the exocrine glands. The immunopathological findings in the thyroid glands of autoimmune thyroid diseases (AITD) and sialadenitis in SS show similarities such as infiltration by activated T cells and expression of HLA-class II molecules on the epithelial cells. To clarify the relationship between SS and AITD, we evaluated the prevalence of AITD in SS patients and that of SS in AITD patients. Chronic thyroiditis was clinically diagnosed in 23 (28%) of 82 patients with SS. Although 16 patients were euthyroid, hypothyroidism was found in 5 cases and hyperthyroidism in one case. The features of SS were found in 12 (27%) of 45 patients with CT and in one of 24 patients with Graves' disease. These results suggest that a common immunological mechanism may be involved in the development of AITD and SS.