The study is designed to examine the relative abundance and activity ofThl 7 cells and natural Treg cells in primary antiphospholipid syndrome (APS).Blood samples were collected from 36 adult patients with APS.The frequencies of Thl7 cells and natural Treg cells in the peripheral bloodmononuclear cells (PBMCs) were analyzed by flow cytometry, and transcription factors and cytokines related to Th17 and Treg development and activity were assessed by real time reverse transcription-polymerase chain reaction.PBMCs stained for IL-17 were also analyzed using confocal microscopy.The activity of Th17 cells on human umbilical vein endothelial cells (HUVECs) was studied in vitro.We demonstrated the presence ofTh1 7 cells in PBMCs of patients with APS.Both the percentage of circulating Th17 cells and the related transcription factors (IL-17A/F, ROR γ t, IL-23R)and cytokines (IL-17A, IL-6) significantly, while Treg frequencies and related transcription factors(Foxp3) and cytokines (TGF-β) were down-regulated significantly in APS patients.The percentage of circulating Th17 cells was higher in APS patients at recurrent attack (RA-APS) than those with APS at first attack (FA-APS), while Treg was lower in patients with RA-APS than those with FA-APS.After treatments with anticoagulant, glucocorticoid and IVIG, the percentage of circulating Th17 cells and Treg were partially restored.IL-17A from sera or PBMC culture supernatants of APS patients could induce cell adhesion molecule mRNA expression in HUVECs.These results suggest that an increased Th 17/Treg ratio exists due to a distinct cytokine environment in APS patients.Thus, the imbalance of Th17/Treg cells might play a role in the immunopathogenesis of APS.