Autoimmune diseases of the thyroid gland are among the most frequent endocrine disorders. The present study analyzes expression patterns of sialic acids in these diseases. Three lectins specific for sialic acids were used for the histochemical analysis of surgical specimens of the thyroid gland: Tritrichomonas mobilensis lectin that stains all types of sialic acids, Maackia amurensis leukoagglutinin that stains sialic acids with alpha2,3 linkage and Sambucus nigra agglutinin that stains sialic acids with alpha2,6 linkage. In autoimmune thyroiditis, there was a significant increase in sialic acid expression in epithelial cells, especially on luminal membranes of follicular cells. The alpha2,3 linkage dominated over the alpha2,6 linkage. Lymphocytes of patients with Hashimoto thyroiditis, especially in germinal centers, showed strong expression of alpha2,6-linked sialic acids on their cell membrane. Vascular endothelium was positive in all specimens. It can be concluded, that there is a significant increase in sialic acid expression in autoimmune diseases of the thyroid gland, predominantly of sialic acids with alpha2,3 linkage, whereas the sialylation pattern of lymphocytes in Hashimoto thyroiditis was also different.