AIM The aim of this study was to assess the vitamin D serum level in women with ovarian endometriosis; specifically, a possible correlation between the dimensions of ovarian endometriomas and vitamin D serum levels was evaluated. MATERIALS AND METHODS This was an observational study of childbearing-age women diagnosed with singleton ovarian endometrioma from January 2015 to December 2015. Women diagnosed with multiple ovarian endometriomas or extraovarian endometriosis were excluded. RESULTS Forty-nine women constituted the initial study cohort. In these women, the mean (±SD) 25-OH-D3 serum level was 22.0 (±8.9) ng/ml, and 42 of them (85.7%) were diagnosed with hypovitaminosis D. In the "hypovitaminosis D women", the mean (± SD) diameter of ovarian endometriomas was 40.2 ± 22.6 mm, while in the "normal vitamin D serum level women" it was 26.7 ± 12.1 mm (p = 0.1). However, a significant linear correlation between 25-OH-D3 serum level and the diameter of ovarian endometriomas was found (r = -0.3, p = 0.03). CONCLUSION We found a relatively high rate of women with ovarian endometriosis and hypovitaminosis D. Interestingly, a significant linear correlation between 25-OH-D3 serum levels and the diameter of ovarian endometrioma emerged.