Chen X, Diao L, Lian R, et al. - Among women with recurrent implantation failure (RIF), researchers investigated the modulatory impacts of vitamin D on peripheral blood and endometrial cellular immunity. The participants were recruited from a fertility center between January 2018 and March 2019, and included 154 women with RIF. During the mid‐luteal phase prior to IVF treatment or pregnancy, samples of blood and endometrium were obtained. Prior to as well as following vitamin D supplementation, serum vitamin D status, NK cell cytotoxicity, Th1 cytokine production, and endometrial immune cells were assessed. In vitamin D insufficiency group (VDI), a significantly higher NK cell cytotoxicity at an effector:target (E:T) ratio of 50:1 or 25:1 was seen, relative to those in vitamin D normal group (VDN). Compared with VDN, a significantly higher percentage of CD68⁺ macrophages on all endometrial cells was noted in VDI and vitamin D deficiency group, while no significant disparities in the percentage of other endometrial immune cells among the three groups were evident. Vitamin D supplementation led to significant decrease in this dysregulation. Overall, findings are suggestive of a likely crucial role of vitamin D in the regulation of systemic as well as local immune response for optimization of maternal tolerance for implantation in women with RIF. Considering pre‐conception optimization of vitamin D status in women with RIF was recommended.