Objective To study the expression of co-stimulatory molecules, GL50-ICOS, in thyroid tissue of patients with Graves' disease (GD) and to explore their relationship with the immune pathogenesis of GD.Methods RT-PCR, Western blot, immunohistochemistry were applied to detect the expression of GL50-ICOS in thyroid of GD. Thyrocytes were cultured in the absence or presence of pro-inflammatory cytokines. The expression of GL50 on thyroid follicular cells (TFC) was further measured by flow cytometry. Results (1) In GD patients,the percentage of CD4+ CD28- T cells was significantly increased as compared with the control healthy individuals. The expression of co-stimulatory molecule ICOS was up-regulated. (2) The mRNA level of ICOS was significantly increased in GD patients than that in nontoxic goiter(NTG) patients(P＜0.01). (3)Compared with NTG control group, the GL50 protein expression was much higher in thyroid tissues of GD patients (P ＜0.01). (4)The results of immunohistochemistry showed that GL50 expression was observed in all GD thyroid tissues, while no expression of GL50 was detected in NTG thyroid tissues(P＜0. 01). (5) The expression of GL50on primary cultured thyroid follicular cells was significantly increased under the stimulatation of pro-inflammatory cytokines in vitro. Conclusion GL50-ICOS is expressed abnormally in thyroid tissue of patients with GD.