To investigate the expression and distribution of S-100 protein and CD83 in the thyroid tissues of autoimmune thyroid diseases (ATDs), and to study the role of the dendritic cells in the pathogenesis of ATDs,immunohistochemical staining was used on pathological tissues of 20 patients with Hashimoto's thyroiditis (HT)and 20 patients with Graves' disease (GD) to check the expression and distribution of S-100 protein and CD83.Compared with control group (20 cases of thyroid follicular adenoma, TFA), the higher expressions of S-100 in HT (139.38±5.92 vs 59.47±11.69) and GD (119.42±14.48 vs 59.47±11.69) were observed respectively (p＜0.001). The increased positive expressions of CD83 which is known as a marker of mature and activated DCs in HT (22.58±13.96 vs 5.19±8.08) and GD (29.92±14.43 vs 5.19±8.08) were also found respectively (p＜0.001).Serum TPO antibody (TPO-Ab, 67.3 ± 11.6%) and Tg antibody (Tg-Ab, 59.8±10.1%) in HT were higher than that in GD (28.4±5.7%, 23.1±4.9%) and that in TFA (6.1±3.4%, 7.2±4.6%)(p＜0.01). Serum TR-Ab in GD(16.3±5.6 U/L) was higher than that in HT (4.8±2.3 U/L) and that in TFA (2.5±1.2 U/L) (p＜0.01). Our findings suggest that the high expression of DCs' markers may be related to the pathogenesis of HT and GD.The upregulation of both number and matured functions of DCs, may lead to present more antigens and to produce more auto-antibodies (such as TgAb and TPOAb in HT, TRAb in GD), which may be involved in pathogenesis of the autoimmune thyroid diseases. Cellular & Molecular Immunology. 2004; 1(5):378-382.