OBJECTIVE To assess the correlation between the levels of vitamin D in follicular fluid and serum, and to determine whether the level of 25-hydroxyvitamin D (25OH-D) in follicular fluid of infertile women undergoing assisted reproductive technology (ART) is associated with the outcome. METHODS Eighty-two infertile women undergoing ART at an academic tertiary care centre were recruited for a prospective cohort study. Levels of 25OH-D in follicular fluid and serum were measured. Standardized regimens for pituitary downregulation and controlled ovarian hyperstimulation were employed. Patient and cycle parameters, and clinical pregnancy (defined as evidence of intra-uterine gestation sac plus heart rate on ultrasound) were determined. RESULTS A significant correlation was found between the levels of vitamin D in follicular fluid and serum (r=0.767, p=0.001). The overall rates of chemical, clinical and ongoing pregnancy were 35.5% (n=29), 29.3% (n=24) and 23.2% (n=19), respectively. No significant difference was found in these pregnancy rates between the tertiles of 25OH-D level in follicular fluid (p=0.959, 0.995 and 0.604, respectively). The median serum level of vitamin D was 8.13 (range 5.37-13.62)ng/ml in the clinically pregnant group and 8.29 (range 5.93-21.23)ng/ml in the non-pregnant group (p=0.235). Interestingly, the median level of vitamin D in follicular fluid was 9.19 (range 5.25-19.51)ng/ml in the clinically pregnant group and 10.34 (range 5.89-29.69)ng/ml in the non-pregnant group (p=0.433). The fertilization rate decreased significantly and the implantation rate increased (not significantly) with increasing tertiles of 25OH-D level in follicular fluid. CONCLUSIONS The level of 25OH-D in follicular fluid is reflective of body stores of vitamin D. Most subjects in this study were deficient in vitamin D, but this study found that vitamin D deficiency does not play a pivotal role in the outcome of ART.