en Obstetrics & Gynicology Obstetrics & Gynicology Original Article Original Article <span style="font-size:14px;"><span style="font-family:times new roman;"><span style="color:#0000FF;"><strong><em>Background</em></strong><em>:</em></span> Uterine leiomyomas (fibroids) are the most common benign pelvic tumors in women of reproductive age with an incidence ranging from 5.4% to 77%, Also, there is a high prevalence of vit D deficiency in Iran and there are the numbers of in vivo and vitro biological studies on the relationship of vitamin D and uterine leiomyomas^. The aim of this study was to evaluate the effect of vit D supplementation on the size of uterine leiomyoma in women with vit D deficiency.<br> <span style="color:#0000FF;"><strong><em>Methods</em></strong><em>:</em></span> This double-blind prospective clinical trial was performed on 69 patients with uterine leiomyomas who had vit D deficiency. Group A (n=35) was treated with vit D 50,000 IU every 2 weeks for 10 weeks, while group B (n=34) received placebo with same color and shape. Finally, the leiomyoma size in both groups was compared (IRCT: 20160521027998N5).<br> <span style="color:#0000FF;"><strong><em>Results</em></strong><em>:</em></span> After a 10-week intervention, 25-hydroxyvitamin D₃ levels were significantly higher in group receiving vitamin D (36.08 vs 16.25 ng/ml). (P<0.001) Leiomyomas size in vit D group significantly decreased as compared to placebo group (52.58 vs 61.11 mm, respectively).<br> <span style="color:#0000FF;"><strong><em>Conclusion</em></strong><em>:</em> </span>Our results showed that administration of vit D3 may reduce the size of leiomyoma. It seems that vitamin D administration is the effective way to treat leiomyoma.</span></span> Uterine leiomyomas, 25-hydroxyvitamin D3, vitamin D deficiency 125 131 http://caspjim.com/browse.php?a_code=A-10-1132-1&slc_lang=en&sid=1 Maryam Hajhashemi hajhashemy73@yahoo.com 100319475328460016766 100319475328460016766 No Department of Obstetrics and Gynecology, Faculty of medicine, Isfahan University of Medical Sciences, Isfahan, Iran Maryam Ansari drm_ansari@yahoo.com 100319475328460016767 100319475328460016767 No Student Research Committee, Faculty of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran Fedyeh Haghollahi fedyeh_hagh@yahoo.com 100319475328460016768 100319475328460016768 Yes Vali ASR Reproductive Health Research Center, Tehran University of Medical Sciences, Tehran, Iran Bita Eslami dr.bes.96@gmail.com 100319475328460016769 100319475328460016769 No Breast Disease Research Center (BDRC), Tehran University of Medical Sciences, Tehran, Iran