To investigate plasma IL-17 level and the expression of Th17 cell transcription factor RORγt in the pathogenesis of Behçet's Disease (BD).Blood samples were collected from 73 patients with BD (45 patients were in active stage), 20 systemic lupus erythematosus (SLE) and 12 multiple sclerosis patients (MS). Twelve patients with BD were investigated both in their active and remission stages. Samples were processed to detect IL-17A level in plasma by enzyme-linked immunosorbent assay (ELISA). Related gene expression was assessed by real-time reverse transcription polymerase chain reaction. Function of Th17 cells in active BD patients with erythema nodosum (EN)-like eruption was studied in relation to human umbilical vein endothelial cells (HUVECs).We demonstrated the presence of Th17 cells and RORγt among the peripheral blood mononuclear cells (PBMC). The percentage of circulating Th17 cells and the ability to produce interleukin-17A (IL-17A) were increased in samples derived from patients with active BD, MS and SLE patients. We observed that IL-17A from patients with active BD could induce adhesion molecule messenger RNA expression in HUVECs.RORγt determined Th17 cell might be involved with increased IL-17A in BD. Our results indicate that IL-17 contributes to the active proinflammatory pattern that is characteristic of inflammatory diseases and patients with active BD.