Objective: To evaluate the effect of vitamin D supplementation on patients with PCOS. Methods: We performed a literature search in database and identified all of the RCTs published before December 2015 that compared the effect of vitamin D supplementation with placebo or metformin in PCOS patients. Main results: Nine out of 463 identified studies were included, involving 502 women presenting with PCOS. Vitamin D supplementation had significant effect on the improvement of follicular development with a higher number of dominant follicles (OR, 234; 95% CI, 1.39 to 3.92). Differences in regular menstrual cycles were also observed when metformin plus vitamin D was compared with metformin alone (OR, 1.85; 95% CI, 1.01 to 3.39). Conclusions: Evidence from available RCTs suggests vitamin D supplementation may be beneficial for follicular development and menstrual cycle regulation in patients with PCOS. Additional high-quality RCTs are required to confirm the effectiveness of vitamin D on PCOS. (C) 2016 Elsevier Ltd. All rights reserved.