Objective To detect the expressions of miR-155,T helper type 17 (Thl7) cells,and Th17 cellspecific transcription factor RORγt and effector cytokine interleukin (IL)-17 in peripheral blood and skin lesions from,and to evaluate their relationship in,patients with atopic dermatitis (AD).Methods Peripheral blood was obtained from 37 patients with AD and 33 age-and sex-matched healthy controls,and biopsy specimens from the lesional and perilesional skin of five patients with severe AD as well as from the normal skin of five healthy human controls.Real-time fluorescence-based reverse transcription (RT)-PCR was employed to measure the mRNA expression levels of miR-155,RORγt and IL-17 in peripheral blood mononuclear cells (PBMCs) and skin specimens,flow cytometry to detect the percentage of Th17 cells in PBMCs,enzyme-linked immunosorbent assay (ELISA) to determine the plasma concentration of IL-17.Statistical analysis was done using independent sample's t test,one-way analysis of variance followed by the least significant difference test,and linear correlation analysis.Results Compared with the healthy controls,the patients with AD showed a significant increase in Th17 cell percentage (1.78％ ± 0.52％ vs.0.47％ ± 0.15％,P＜ 0.01),mRNA expression levels of miR-155 (5.78 ± 1.78 vs.1.82 ± 0.46,P＜ 0.01),RORγt (6.08 ± 1.04 vs.1.64 ± 0.52,P＜ 0.01) and IL-17 (7.09 ± 1.75 vs.1.71 ± 0.46,P＜ 0.01),as well as in the plasma concentration of IL-17 ((2.51 ± 6.15) pg/ml vs.(11.80 ± 2.24) pg/ml,P＜ 0.01).There was a sequential decrease in the expression levels of miR-155,RORγt and IL-17 mRNA from lesional skin,perilesional skin to normal skin (F =41.803,17.040 and 37.064 respectively,all P ＜ 0.01).The miR-155 mRNA expression level in PBMCs was positively correlated with the SCORing Atopic Dermatitis (SCORAD) index,Th17 cell percentage,RORγt and IL-17 mRNA expression levels as well as IL-17 plasma concentration (r =0.405,0.426,0.402,0.410 and 0.408 respectively,all P ＜ 0.05).Similarly,the miR-155 expression level was positively correlated with RORγt and IL-17 mRNA expression levels in lesional and paralesional specimens (r =0.428 and 0.435 respectively,both P ＜ 0.05).Conclusion The up-regulated expression of miR-155,Th17 cells and their effector cytokine IL-17 may be associated with the development of AD.