To investigate the expression and distribution of S-100 protein and CD83 in the thyroid tissues of autoimmunethyroid 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 inHT(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 inHT(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 thanthose in GD(28.4±5.7%,23.1±4.9%) and 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 those in HT(4.8±2.3 U/L) and TFA(2.5±1.2 U/L)(p<0.01).Our findings suggestthat the high expression of DCs’ markers may be related to the pathogenesis of HT and GD.The upregulationof both the number and the matured functions of DCs,may lead to present more antigens and to produce moreauto-antibodies(such as Tg-Ab and TPO-Ab in HT,TR-Ab in GD),which may be involved in pathogenesis ofthe autoimmune thyroid diseases.Cellular & Molecular Immunology.2004;1(5):378-382. 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 inHT (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 are known as a marker of mature and activated DCs inHT ( Serum TPO antibody (TPO-Ab, 67.3 ± 11.6%) and Tg antibody (Tg-Ab, 59.8 ± 10.1%) in HT were higher thanthose in GD (28.4 ± 5.7%, 23.1 ± 4.9%) and TFA (6.1 ± 3.4%, 7.2 ± 4.6%) (p <0.01) .Serum TR-Ab in GD was higher than those in HT (4.8 ± 2.3 U / L) and TFA (2.5 ± 1.2 U / L) (p <0.01) suggestthat the high expression of DCs’ markers may be related to the pathogenesis of HT and GD. upregulationof both the number and the matured functions of DCs, may lead to present more antigens and to produce moreauto-antibodies (such as Tg-Ab and TPO-Ab in HT, TR-Ab in GD, which may be involved in pathogenesis of the autoimmune thyroid diseases.Cellular & Molecular Immunology. 2004; 1 (5): 378-382.