Decidualisation of endometrial stromal cells (ESC) is a prerequisite for the implantation of human embryos. Identification of genes that are upregulated or downregulated during decidualisation could lead to a better understanding of the molecular mechanisms involved in this process. In the present study, we examined differences in gene expression between decidualised and non-decidualised cells using microarray analysis and found that Factor XII (FXII) gene expression was upregulated during decidualisation. Furthermore, we also examined the expression of FXII by human ESC before and during pregnancy, as well as its expression by cells that had undergone decidualisation in vitro. Weak expression of FXII mRNA was detected in the non-pregnant endometrium that increased gradually from the proliferative to the secretory endometrium. During pregnancy, FXII mRNA expression was markedly increased in decidualised endometrium. When sex steroids (200 pg mL^–1 of 17β-oestradiol and 100 ng mL^–1 of progesterone) were used to induce in vitro decidualisation of ESC, the expression of FXII mRNA increased by approximately 25.3-fold compared with that in non-decidualised ESC. Using western blotting, we confirmed the presence of FXII protein (80 kDa) in ESC after in vitro decidualisation. Increased expression of FXII in ESC during decidualisation suggests that the kallikrein–kininogen–kinin system may be activated during the implantation of human embryos.