Background: polycystic ovary syndrome (PCOS) is the most common endocrine disorder in women of reproductive age, with a prevalence of 6–10% in the general population. PCOS is characterized by the following: ovulatory dysfunction resulting in oligo-amenorrhea and/or anovulation, hyperandrogenism and/or hirsutism and the presence of polycystic ovarian morphology by ultrasound. Vitamin D also plays a physiologic role in reproduction including ovarian follicular development and luteinization via altering anti-Müllerian hormone (AMH), signaling, follicle-stimulating hormone sensitivity and progesterone production in human granulosa cells. Aim of the Work: this study aimed to compare levels of vitamin D in women with PCOS and healthy fertile women. Patient and Methods: this case control study was conducted in Ain Shams Maternity hospital in outpatient infertility clinics and family planning clinic during the period from March 2017 to October 2017. It was included 40 women diagnosed with PCOS based on Rotterdam criteria and 40 fertile women without PCOS who were recruited from family planning clinic. Results: 25(OH) vitamin-D level was significantly lower in PCOS group than in the control group. Conclusion: 25(OH) vitamin-D was significantly lower in PCOS group than in the control group, but not sufficient to be a diagnostic tool for PCOS. Recommendation: larger studies are still needed to clarify the rule of vitamin D in infertility.