Background: This study investigated the effects of interleukin 23 (IL-23) on the production of cytokines (IL-1, IL-4, IL-10, and IL-17), the differentiation of Treg/Th17 and STAT3 (i.e., signal transducer and activator of transcription 3) in human decidual immune cells (DICs) during early pregnancy. Methods: DICs were treated with recombinant human IL-23 and an antibody against IL-23 subunit p19. The differentiation of Treg and Th17 cells was detected by flow cytometry. Levels of IL-23 receptor (IL-23R), STAT3, and phosphorylated STAT3 (pSTAT3) was examined by Western blot. The production of ILL IL4, IL10, and IL-17 in DICs was measured by ELISA. Results: Exogenous recombinant human IL-23 significantly promoted the differentiation of Th17 cells from DICs, while anti-IL-23 antibody significantly promoted the differentiation of Treg cells from DICs. Consistent with the differentiation of Th17 and Tregs cells, levels of IL-1 beta and IL-17 correlated positively with IL-23 treatment, and anti-IL-23 antibody increased the secretion of IL-4 and IL-10 from DICs. Levels of pSTAT3, but not STAT3 or IL-23R, were significantly elevated by recombinant IL-23 treatment; anti-IL-23 antibody significantly decreased the levels of pSTAT3 and IL-23R in DICs. Conclusions: IL-23 mediates the differentiation of Th17 and Treg cells and the production of associated cytokines in DICs. The potential mechanism likely involves the STAT3 pathway. (C) 2015 The Authors. Published by Elsevier Inc.