Objectives: Evaluation of the effect of 12-wk vitamin D supplemental therapy (VDST) on serum levels of 25-hydroxy vitamin D (25OH-VD) and endometriosis-related pain scores in women with endometriosis Patients & Methods:41 women with endometriosis were evaluated for presence and severity of dysmenorrhea, dyspareunia and non-menstrual pelvic pain and gave blood samples for estimation of serum 25OH-VD level. All women received VDST as 5000 units/day for 12 weeks and clinical pain scores and serum 25OH-VD were re-evaluated. Results: After VDST, median value of total pain score was decreased by 25% and the frequency of patients had mild pain was increased by 1.5 folds, while the frequency of patients had severe pain was decreased by 50%. Moreover, the frequency of patients dependent on gabapentin alone or with injectable NSAID was reduced by 75% with concomitant increased frequency of patients dependent on oral NSAID by 2.2 folds. Furthermore, VDST increased serum 25OH-VD levels by 25% with concomitant decreased frequency of women had VD deficiency (VDD) by 1.3 folds. There was negative significant correlation between change of serum 25OH-VD levels and total pain score. Conclusion: VDD is widespread between women with endometriosis. VDST for 12-wk improved VD sufficiency status and endometriosis-related pain scores.