Intrauterine devices (IUDs) exert contraceptive action by interfering with sperm transport, ovum development, fertilization and implantation. Glycodelin A (GdA) is a uterine glycoprotein that has local contraceptive activity by inhibiting sperm-egg binding. GdA is normally absent from endometrium during the fertile midcycle and it is not expressed until the fifth postovulatory day. The phase of menstrual cycle addressed in this study covers the phase when conception is most likely to follow an unprotected intercourse and when GdA is normally absent. We present here evidence that levonorgestrel-releasing IUD (LNg-IUD) is accompanied by 'inappropriate' expression of GdA in endometrium between days 7 and 16 of the menstrual cycle (six out of six cases). The same was also found in copper-releasing IUD (Cu-IUD)-wearing women, but less frequently (four out of 11 cases, P < 0.0345, Fisher's exact test). In-situ hybridization localized GdA mRNA into endometrial glands in the midcycle endometrium, confirming the cellular site of synthesis. Based on the potent inhibitory activity of GdA on sperm-egg binding, the presence of GdA in uterine glands of IUD wearers may lead to prior exposure of sperm to contraceptive GdA, thus contributing to the contraceptive activity of the IUD.