From a total of 63 patients with autoimmune thyroiditis, 19 cases were further investigated to determine the degree of concomitant morphologic and functional salivary gland changes. For comparison, 21 of a total of 28 cases of primary Sjögren's syndrome were also examined. Of the 19 cases of autoimmune thyroiditis, 11 showed various degrees of salivary gland involvement on the basis of an analysis of lower lip salivary gland biopsy specimens, scintigraphy of the parotid, and unstimulated whole sialometry. Six of these cases fulfilled the criteria of primary Sjögren's syndrome. A remarkably high proportion of dark-staining acini was observed in the lower lip biopsy specimens of our patients with thyroiditis (8 of 19, 42%) and less among our patients with primary Sjögren's syndrome (5 of 21, 24%). We conclude that significant involvement of salivary glands may occur in cases of autoimmune thyroiditis, which indicates that common mechanisms may frequently be operative in the development of thyroid and salivary gland immune disease.