Background: Vitamin D deficiency is a potentially modifiable risk factor that may be targeted for breast cancer(BC) prevention. It may also be related to prognosis after diagnosis and treatment. The aim of our study was todetermine the prevalence of vitamin D deficiency as measured by serum 25-hydroxy vitamin D (25-OHD) levelsin patients with BC and to evaluate its correlations with life-style and treatments. Materials and Methods: Thisstudy included 186 patients with stage 0-III BC treated in our breast center between 2010-2013. The correlationbetween serum baseline 25-OHD levels and supplement usage, age, menopausal status, diabetes mellitus, usageof bisphosphonates, body-mass index (BMI), season, dressing style, administration of systemic treatments andradiotherapy were investigated. The distribution of serum 25-OHD levels was categorized as deficient (<10ng/ml), insufficient (10-24 ng/ml), and sufficient (25-80 ng/ml). Results: The median age of the patients was 51 years(range: 27-79 years) and 70% of them had deficient/insufficient 25-OHD levels. On univariate analysis, vitaminD deficiency/insufficiency was more common in patients with none or low dose vitamin D supplementation at thebaseline, high BMI (≥25), no bisphosphonate usage, and a conservative dressing style. On multivariate analysis,none or low dose vitamin D supplementation, and decreased sun-exposure due to a conservative dressing stylewere found as independent factors increasing risk of vitamin D deficiency/insufficiency 28.7 (p=0.002) and 13.4(p=0.003) fold, respectively. Conclusions: The prevalence of serum 25-OHD deficiency/insufficiency is high in ourBC survivors. Vitamin D status should be routinely evaluated for all women, especially those with a conservativedressing style, as part of regular preventive care, and they should take supplemental vitamin D.