Objective: The aim of this study is to evaluate the effects of 25-OH vitamin D on pregnancy outcomes in infertile patients undergoing a frozen embryo transfer. Methods: In this prospective, single-blind study conducted at Kocaeli University Medical Faculty Hospital, Center for Assisted Reproductive Techniques, baseline serum levels of 25-OH vitamin D were measured at the start of treatment in 276 infertile patients who were scheduled to undergo frozen embryo transfer (FET). Cases with 25-OH-D vitamin levels lower than the level of deficiency (<20 ng/ml, group A, n=48) and higher than the level of deficiency (≥20 ng/ml, group B, n=44) were compared in terms of the rates of pregnancy as an outcome of the FET cycle, clinical pregnancy, ongoing pregnancy, live birth, implantation, pregnancy loss, and multiple pregnancy. Results: Cases in groups 1 and 2 had similar demographic characteristics, and the serum AMH levels, one of the cycle followup parameters, were statistically significantly higher in group 1 compared to group 2 (p=0.014). Pregnancy (41.6% vs. 31.8%), clinical pregnancy (35.4% vs. 25%), ongoing pregnancy (25% vs. 18.2%), live birth (20.8% vs. 18.2%), pregnancy loss (18.8% vs. 13.6%) and twin pregnancy (4.2% vs. 9.1%) were similar between the groups (p=0.328, p=0.278, p=0.428, p=0.749, p=0.507, p=0.421, respectively). Conclusion: There was no correlation between pregnancy outcomes from frozen embryo transfer and baseline serum 25-OH vitamin D levels obtained at the start of treatment. There is no indirect evidence showing that vitamin D level exerts its effects on fertility through endometrial receptivity and the implantation process.