Background: To evaluate the effects of vitamin D (vitD) supplement on uterine fibroid growth., Materials and Methods: A randomized blinded clinical trial was conducted at a tertiary university-based hospital from August 2017 to September 2018. Totally, 204 women were enrolled into the study. They had at least one uterine fibroid >10 mm on transvaginal ultrasound and their vitD level was insufficient (i.e. 20-30 ng/ml). The intervention group was treated with vitD 50000 U supplements for two months. After 2 months, ultrasound screening and vitD level measurement was done in both groups., Results: At first, the mean serum vitD levels in intervention and control group were 23.62 and 23.20 ng/ml, respectively. After 8 weeks, the mean serum vitD levels in the control and intervention group were 22.72 and 28.56 ng/ml respectively (P<0.05). Also, mean fibroma diameter in the intervention group before and after 8 weeks of vitD supplementation was 43 ± 4.68 and 42.6 ± 1.31 mm, respectively. Mean uterine fibroid diameter in the control group which did not receive vitD supplements, before and after 8 weeks was 41.98 ± 5.25 and 47.81 ± 3.42 mm, respectively. The variation in the mean size of the uterine fibroid between the control and intervention group which was respectively about 5.83 mm increase and 0.48 mm decrease, was significant (P<0.001)., Conclusion: Our results showed that vitD supplementation prevents fibroid growth. It seems that vitD supplement is a simple, safe and inexpensive modality for leiomyoma growth prevention (Registration number: IRCT201703122576N15)., Competing Interests: The authors declare no conflicts of interest., (Copyright© by Royan Institute. All rights reserved.)