In order to elucidate the function of homeobox A10 gene (HOXA10) and p57 during decidualization our present study was designed to observe the change of HOXA10 and p57 expression and subcellular localization of HOXA10 in the process of endometrial stromal cell (ESC) differentiation in vitro. Decidualization was induced by 0.5 mmol/L 8-Bromo-cAMP (8-Br-cAMP) together with 1x10(-6) mol/L medroxyprogesterone acetate (MPA). Expression of p57 and HOXA10 was detected by RT-PCR and Western blot after 1-day, 2-day, and 4-day treatment (D1, D2, D4). ESCs cultured in 2%FBS for 1 and 4 d were used as control (C1, C4). The location of HOXA10 was detected by indirect immunofluorescence and HOXA10-GFP transfection. The results are as follows: (1) The expression of HOXA10 decreased progressively during the course of decidualization, and showed significant difference compared to control group C4 after 2-day treatment (D2). (2) On the contrary, the expression of p57 increased progressively and also showed significant difference compared to the control group C4 after 2-day treatment (D2). (3) There was no significant change of HOXA10 and p57 expression after culturing ESCs in 2%FBS for 4 d (C1, C4). (4) HOXA10 located in the nucleus throughout the course. Cytoplasm and nucleus shuttle was not detected in the experiment. Our results suggest that the down-regulation of HOXA10 may contribute to the increase of p57 and that the up-regulation of p57 likely plays an important role in ESC differentiation. Progesterone receptor (PR) pathway may participate in promoting ESCs to exit cell cycle and enter differentiation.