Summary Introduction: HOXA genes are receptivity markers and their altered expression can help identify women with implantation defect. Objectives: The purpose of this study was to examine the expression pattern of HOXA11 and expression and localization of its protein product in the endometrium of women with endometriosis, idiopathic infertility and normal fertile patients during different phases of the menstrual cycle. Material and methods: We evaluated HOXA11 mRNA level in the endometrium from endometriosis (n=36), idiopathic infertility (n=48) and fertile patients (n=30) during a menstrual cycle. The amounts of mRNA were determined by real-time quantitative PCR. Using the immunohistochemical techniques we compared the localization of HOXA11 protein in the proliferative, early secretory and midsecretory endometrium in all of the studied groups. Endometrial biopsy was performed by pipelle or during hysteroscopy. Results: We observed statistically significantly elevated HOXA11 transcripts levels in the midsecretory phase in both, the idiopathic infertility and the fertile control groups (p<0.05). Patients with endometriosis had low HOXA11 transcript level in the implantation window. Normal fertile patients showed statistically significantly decreased (p=0.003) HOXA11 mRNA level in the proliferative phase and statistically significantly increased level (p=0.018) in midsecretory phase, comparing to endometriosis patients. HOXA11 protein were localized in the nuclei of the endometrial stromal cells whereas the cytoplasm of these cells did not stain. Conclusion: Our results suggest that altered HOXA11 gene expression in the endometrium during a menstrual cycle may be a common phenomenon among patients with endometriosis and may cause infertility in this group of patients. Further research should explain the mechanism of altered expression of HOXA11 gene.