Intercellular adhesion molecule, a ligand for the leucocyte integrins CD11a/CD18 (LFA-1) and CD11b/CD18 (Mac-1), that plays an important role in a variety of inflammatory and immune-mediated mechanisms, is strongly expressed in retroocular connective tissue from patients with Graves' ophthalmopathy (GO) and involved in lymphocyte attachment to cultured retroocular fibroblasts via the ICAM-1/LFA-1-mediated pathway. Here, we report the detection and functional activity of a soluble form of the ICAM-1 molecule (sICAM-1) in sera from patients with GO and other thyroid diseases. Serum concentrations for sICAM-1 were determined using a highly sensitive ELISA. Compared with normal controls, patients with hyperthyroid or euthyroid GO and patients with Riedel's invasive fibrous thyroiditis revealed markedly elevated sICAM-1 serum concentrations (all P < 0.0001). In patients with Graves' disease (GD) without clinical GO and in patients with Hashimoto's thyroiditis (HT), sICAM-1 levels were elevated to a lesser degree (both P < 0.001). sICAM-1 serum levels in patients with non-autoimmune hyperthyroidism due to a toxic adenoma were not significantly different from normal controls. In a separate group of 12 patients with severe inflammatory GO, sICAM-1 serum levels markedly declined (P < 0.0001) within 3 months of glucocorticoid therapy in nine patients who responded to this form of treatment with a decrease in periorbital inflammation. In contrast, sICAM-1 serum levels remained unchanged in three patients with poor response to steroids and persistent inflammatory periorbital disease. When tested in a cell adhesion assay, GO sera containing elevated concentrations of sICAM-1 were found to enhance the attachment of peripheral blood mononuclear cells (PBMC) to interferon-gamma (IFN-gamma)-treated retroocular fibroblasts in a dose-dependent manner, up to a maximal stimulation of approximately 5.5-fold (P < 0.001). This effect was abolished by preabsorption of sera with a MoAb against ICAM-1 and inhibited, in a dose-dependent manner, by coincubation with increasing concentrations of purified sICAM-1. In conclusion, sICAM-1 concentrations are markedly elevated in sera from patients with GO, and changes in sICAM-1 serum levels during glucocorticoid therapy closely parallel changes in the degree of inflammation. Given the capacity of sICAM-1 to modulate the adhesion of lymphocytes to retroocular fibroblasts in vitro, sICAM-1 may play a role in the ongoing immune process within the connective tissue in GO.