Objective: To evaluate gene and protein expression of steroid receptors, nuclear receptor coregulators, and uterine receptivity markers in midsecretory phase endometria from untreated women with polycystic ovary syndrome (PCOS).Design: Case-control study. Setting: Hospital research unit. Patient(s): Eight patients with PCOS and eight fertile women of similar age to those with PCOS. In-tervention(s): Endometrial samples were obtained from women with PCOS (PCOSE) and normal (NE)- women during the midsecretory phase of the menstrual cycle. Main Outcome Measure(s): Expression studies (immunohistoch-emistry, reverse transcription-polymerase chain reaction [RT- PCR] and Western blot). Result(s): Endometria from PCOS exhibit higher levels of messenger RNA (mRNA) and protein for estrogen receptor α and coactivators than NE. Epithelial cells had a greater expression of progesterone receptor in PCOSE, whereas, no differences were observed in gene and protein expression of the nuclear corepressor (NcoR)- and the antiadhesion molecule mucin type- 1 (MUC- 1) between PCOSE and NE. Immunodetection for the coactivator ARA70 was higher in PCOSE than in NE; in contrast, expression of β 3- integrin in epithelia was lower in PCOSE than in control endometria. Conclusion(s): The higher response to steroid hormones of endometria from untreated PCOS-women induces diminished expression of β 3 integrin, which partially explain implantation failure in PCOS patients. Objective: To evaluate gene and protein expression of steroid receptors, nuclear receptor core molecules, and uterine receptivity markers in midsecretory phase endometria from untreated women with polycystic ovary syndrome (PCOS). Design: Case-control study. Setting: Hospital research unit. Patient ( Eight patients with PCOS and eight fertile women of similar age to those with PCOS. In-tervention (s): Endometrial specimens were obtained from women with PCOS (PCOSE) and normal (NE) - women during the midsecretory phase of the Main Outcome Measure (s): Expression studies (immunohistoch-emistry, reverse transcription-polymerase chain reaction [RT-PCR] and Western blot). Result (s): Endometria from PCOS exhibit higher levels of messenger RNA and protein for estrogen receptor α and coactivators than NE. Epithelial cells had a greater expression of progesterone receptor in PCOSE, whereas, no differences were observed in gene and protein expression of the nuclear corepresso Immunodetection for the coactivator ARA70 was higher in PCOSE than in NE; in contrast, expression of β 3-integrin in epithelia was lower in (NUCR) - and the antiadhesion molecule mucin type- 1 (MUC- 1) between PCOSE and NE. PCOS than in control endometria. Conclusion (s): The higher response to steroid hormones of endometria from untreated PCOS-women induces diminished expression of β 3 integrin, which partially explain the implantation failure in PCOS patients.