Abstract： Objective To describe the characteristic of Th17 cells in peripheral blood (PB) and bronchoalveolar lavage fluid (BALF) from patients with sarcoidosis,and to investigate the effect of glucocorticoid therapy on the expression of RORγt mRNA,and therefore to explore the role of Th17 cells in the immunopathogenesis of pulmonary sarcoidosis.Methods Ten patients with active pulmonary sarcoidosis who were prescribed with prednisone according to the guidelines were defined as the study group.All of them had positive pathological results and had definite response to corticosteroids. Ten healthy controls were recruited from volunteers with similar sex distribution and age. The proportion of CD4+ IL-17A + T cells in peripheral blood mononuclear cells (PBMC) and BALF were calculated by flow cytometry. The mRNA expression of RORc in PBMC was measured by Real-time PCR.Results The mean age was (52 ± 8) years in both the study group and the controls,and there were 9 females and 1 male in both groups.The proportion of CD4+ IL-17A + T cells was higher in the PB of sarcoidosis patients compared to that of the controls [(1.61±1.09)％ vs (0.51 ±0.43)％,t=3.02,P=0.014].For the same patients,the proportion of CD4+ IL-17A +T cells was higher in the BALF compared to that in the PB [ (3.05 ± 1.87)％ vs (1.61 ±1.09 )％,t =2.94,P =0.001 ].After 4 to 6 week therapy with glucocorticoids,all the 10 patients showed definite response. The level of RORc mRNA in the PB did not decrease significantly after treatment [ (0.952 ±0.367) vs (0.168 ±0.272),t =1.76,P =0.057],although a trend was noted.Conclusions Th17 cells was probably involved in the immunopathogenesis of sarcoidosis, and glucocorticoids might modulate the disturbance of Th17 cells in pulmonary sarcoidosis.