Objectives: Evaluation of the relation between serum levels of 25-hydroxy vitamin D (25-OHVD), tumor necrosis factor-α (TNF-α) and interleukin (IL)-4 and outcome of intracytoplasmic sperm injection (ICSI) for infertile women secondary to diminished ovarian reserve (DOR). 
Patients & Methods: 82 infertile DOR women, defined according to Bologna Criteria, were evaluated and gave blood samples for estimation laboratory parameters before and after receiving 12-w VD supplemental therapy (VDST). All women received the ovarian stimulation flexible antagonist protocol, ICSI was performed, and the day-3 embryos were graded. All women who had no fertilization or had embryos of grade P-G2 underwent another session of ICSI after VDST.  
Results: Pre-VDST, 77 had VD deficiency (VDD) and 5 had insufficiency, Post-VDST 65 women had VDD, 14 had insufficiency and 3 women had sufficient level. Post-VDST AFC was increased by 85.3% and serum levels of 25-OHVD, IL-4, and anti-Müllerian hormone were increased by 31.7%, 18.66%, and 6.18%, respectively, while TNF-α levels were decreased by 12.8% with significantly lower frequency of embryos of poor and fair grades. ROC curve analysis defined pre-VDST high serum levels of 25-OHVD and IL-4, and high AFC as positive, while high levels of TNF-α and high BMI as negative predictors for the high quality embryo.  
Conclusion: VDD, obesity, and deregulated immune milieu in direction of inflammation may predispose or aggravate poor ovarian response to OS. VDST improved ovarian function and quality of oocytes with subsequent improvement of embryo quality most probably through equalization of immune milieu to the direction of anti-inflammatory.