Improving invitro fertilization success is an unmet need. Observational studies have suggested that women with deficient or insufficient vitamin D have lower chances of invitro fertilization success, but whether supplementation improves clinical pregnancy rate remains unclear. This study aimed to determine whether oral vitamin D3 supplementation improves clinical pregnancy in women undergoing an invitro fertilization cycle. The "supplementation of vitamin D and reproductive outcome" trial is a 2-center randomized superiority double-blind placebo-controlled trial. Subjects were recruited between October 2016 and January 2019. Participants were women aged 18 to 39 years with low vitamin D (peripheral 25-hydroxyvitamin D of <30 ng/mL), serum calcium of ≥10.6 mg/dL, body mass index of 18 to 25 kg/m2, and antimüllerian hormone levels of >0.5 ng/mL and starting their first, second, or third treatment cycle of conventional invitro fertilization or intracytoplasmic sperm injection. The primary outcome was the cumulative clinical pregnancy rate per cycle. Pregnancies obtained with both fresh or frozen embryo transfers were included. Clinical pregnancy was defined as an intrauterine gestational sac with a viable fetus. The primary analysis was performed according to the intention-to-treat principle and could also include natural conceptions. Secondary outcomes included total dose of gonadotropins used, embryologic variables (number of oocytes retrieved, number of suitable oocytes retrieved, fertilization rate, and rate of top-quality embryos), and clinical outcomes (miscarriage rate and live birth rate). Overall, 630 women were randomized 2 to 12 weeks before the initiation of the invitro fertilization cycle to receive either a single dose of 600,000 IU of vitamin D3 (n=308) or placebo (n=322). Interestingly, 113 (37%) and 130 (40%) women achieved a clinical pregnancy in the treatment and placebo groups, respectively (P=.37). The risk ratio of clinical pregnancy in women receiving vitamin D3 was 0.91 (95% confidence interval, 0.75-1.11). Compared with the placebo, vitamin D3 supplementation did not improve the rate of clinical pregnancy. Exploratory subgroup analyses for body mass index, age, indication to invitro fertilization, ovarian reserve, interval between drug administration and initiation of the cycle, and basal levels of 25-hydroxyvitamin D failed to highlight any clinical situation that could benefit from the supplementation. In women with normal weight with preserved ovarian reserve and low vitamin D levels undergoing invitro fertilization cycles, a single oral dose of 600,000 IU of vitamin D3 did not improve the rate of clinical pregnancy. Although the findings do not support the use of vitamin D3 supplementation to improve invitro fertilization success rates, further studies are required to rule out milder but potentially interesting benefits and explore the effectiveness of alternative modalities of supplementation.