Cultured thyroid epithelial cells can be induced to express intercellular adhesion molecule-1 (ICAM-1, or CD54). However, constitutive follicular expression of ICAM-1 has been reported only in thyroid autoimmunity. We evaluated the expression of ICAM-1 mRNA and protein on thyroid tissue from different autoimmune thyroid diseases, and its relationship with other immunologically relevant surface markers, namely costimulatory molecules of B7 family. Thyroid tissue sections were obtained by surgically removed thyroid glands from 6 patients with Hashimoto's thyroiditis (HT), 6 with Graves' disease (GD) and 3 with multinodular nontoxic goiter. We used in situ hybridization to localize ICAM-1 mRNA, and immunohistochemical analysis by alkaline phosphatase anti-alkaline phosphatase (APAAP) method. We showed a clear hybridization pattern, localized in follicular cells, in sections of glands with HT. The hybridization pattern was far less pronounced in GD: no staining was apparent on follicular cells. These results were strictly consistent with those obtained by means of immunohistochemistry. Moreover, double-staining experiments demonstrated colocalization of ICAM-1 and B7.1 molecules in HT, whereas no B7.1 expression was observed in Graves' or in non-autoimmune thyroid diseases. These data agree with the hypothesis of distinct immunoregulatory phenomena and effector mechanisms in the 2 main autoimmune thyroid diseases.