To characterize possible endometrial defects in infertile women with isolated PCO morphology.Prospective study.An academic hospital with an IVF unit.Women with primary unexplained infertility and isolated PCO, fertile women, and women with infertility secondary to male factor.Thirty-one women (fertile and with male factor infertility) had endometrial sampling across the menstrual cycle. Nine fertile women and 10 infertile women with isolated PCO had sampling on day LH +7, adjusted for histologic dating.Expression of alphavbeta3 and its ligand, osteopontin, were determined using real-time quantitative polymerase chain reaction and immunohistochemistry. In vitro regulation of osteopontin was assessed using the Ishikawa cell line.Cyclic variations revealed a fall in integrin alphavbeta3 mRNA during the secretory phase with concomitant up-regulation of osteopontin mRNA. Immunohistochemistry on day LH +7 demonstrated a significant reduction in expression of osteopontin in the isolated PCO group with no difference in expression of alphavbeta3. In vitro studies confirmed up-regulation of osteopontin by estrogen with no apparent effect of androgen.These results demonstrate an apparent reduction of osteopontin expression, important in cell-cell adhesion, during the window of implantation in infertile women with isolated PCO morphology.