Objective: The immunomodulatory and anti-inflammatory properties of vitamin D may be the basis for understanding the pathogenesis of endometriosis. The aim of this study was to investigate vitamin D status, by measuring serum 25(OH) vitamin D and vitamin D binding protein (VDBP) levels, in women with endometriosis according to severity. Methods: This prospective study of women with mild endometriosis (n=7), advanced endometriosis (n=9) and controls (n=16). Measurements of serum total 25(OH) vitamin D concentrations were performed using the Elecsys Vitamin D Total Kit with the Cobas e602 module; this is an electrochemiluminescent assay with ruthenium-labeled DBP, biotin-labeled vitamin D, and streptavidin-coated microparticles. The levels of bioavailable and free 25(OH) vitamin D were calculated. Concentrations of VDBP were measured by using Human Vitamin D BP Quantikine ELISA kit (R&D Systems, Minneapolis, MN, USA). For group comparisons, we used parametric test (ANOVA, analysis of variance) and non-parametric tests (Kruskal-Wallis test, Chi-square test). Results: In the clinical and laboratory characteristics, gravidarum, parity, serum ESR and CA125 levels showed significant difference between the three groups. Mean (standard deviation) levels of total 25(OH) vitamin D in control, mild and advanced endometriosis were 17.0 (4.7), 14.2 (7.6) and 8.9 (1.7) ng/mL, respectively (P = .006). There were no statistical significant in bioavailable, free 25(OH) vitamin D and VDBP. In the two groups with endometriosis, bioavailable and free 25(OH) vitamin D were decreased in advanced endometriosis group compared with mild endometriosis group and VDBP was increased in advanced endometriosis group compared with mild endometriosis group. Conclusion: The results of this study support an association between vitamin D levels and the severity of the endometriosis. Therefore, vitamin D supplements should be explored as a novel therapeutic strategy for managing the endometriosis.