T helper 17 (Th17) cells and regulatory T (Treg) cells are two distinct subsets of CD4(+) T cells which have opposite effects on inflammation, autoimmunity and immunological rejection of foreign tissue. Treg cells have been shown to be important in maintaining materno-fetal tolerance, but the role of Th17 cells in human pregnancy and pathological pregnancy, especially in relation to unexplained recurrent spontaneous abortion (RSA), has not been investigated. In this study, we showed that the proportion of Th17 cells in the peripheral blood and decidua was significantly higher in unexplained RSA patients compared to normal, early pregnant women. Meanwhile, there was an inverse relationship between Th17 cells and Treg cells in the peripheral blood lymphocytes (PBL) and decidua in unexplained RSA. The expression of Th17 related factors, IL-17, IL-23 and retinoid orphan nuclear receptor (RORC), in PBL and decidua in unexplained RSA patients was significantly higher than normal, early pregnant women. This study is the first to define the occurrence of Th17 cells in unexplained RSA patients and in normal, early pregnant women. We suggest that these highly pro-inflammatory cells contribute to unexplained RSA, and the balance between Th17 cells and Treg cells may be critical to pregnancy outcomes.