Chen X, Diao L, Lian R, et al. - Researchers examined how vitamin D modulates peripheral blood and endometrial cellular immunity in 154 women with recurrent implantation failure (RIF) from January 2018 and March 2019. They collected blood and endometrium samples during the mid‐luteal phase before IVF treatment or pregnancy and assessed serum vitamin D status, NK cell cytotoxicity, Th1 cytokine production and endometrial immune cells before and after vitamin D supplementation. Vitamin D insufficiency group (VDI) vs vitamin D normal group (VDN) had significantly higher NK cell cytotoxicity at an effector‐to‐target (E:T) ratio of 50:1 or 25:1. Compared with VDN, VDI or vitamin D deficiency group (VDD) had significantly increased percentage of IFN‐γ‐ or TNF‐α‐producing Th cells. VDI and VDD exhibited significantly higher percentage of CD68+ macrophages on all endometrial cells than VDN. Findings thereby suggest a significant role of vitamin D in the regulation of not only systemic but also local immune response for optimization of maternal tolerance for implantation in women with RIF. Hence consideration should be given to preconception optimization of vitamin D status in women with RIF.