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Recurrent implantation failure (RIF) is a challenging situation for infertility specialists, and its treatment is introduced as a difficult case in the field of assisted reproductive technology (ART). Vitamin D (VD) is one of the supplements that have been suggested to improve the implantation process. In the present study, the effect of VD on the expression and protein levels of VD receptor (VDR), progesterone receptor (PR), prolactin (PRL), insulin-like growth factor binding protein-1 (IGFBP-1), and homeobox protein A10 (HOXA10) in the endometrial cells of unknown RIF women with and without VD deficiency were assessed by qRT-PCR and immunohistochemistry. Twelve women with unknown RIF and VD deficiency (≤ 20 ng/ml) and twelve women with unknown RIF without VD deficiency (≥ 30 ng/ml) from 2021 to 2022 were identified. Endometrial specimens were collected in the mid-luteal stage before treatment or pregnancy. In the group with VD deficiency, oral medication of VD 50,000 units was prescribed for 2 to 3 months and their serum levels of VD were re-measured, then an endometrial biopsy at the same stage of the menstrual cycle was performed. The expression and protein levels of VDR, PR, PRL, IGFBP1, and HOXA10 in RIF patients with VD deficiency were lower than the RIF patients without VD deficiency (P value < 0.05). Our findings suggest that VD can play a key role in the pregnancy process, especially during embryo implantation and decidualization of the endometrial cells.IRCT registration number: IRCT20220528055006N1, Registration date: 2022-10-15, Registration timing: retrospective., (© 2023. The Author(s), under exclusive licence to Society for Reproductive Investigation.)