Purpose of investigation: The objective was to evaluate serum levels of vitamin D in patients who presented with infertility. Materials and Methods: For this retrospective study, the authors examined medical records of all infertile patients who visited this clinic between March and August 2017, Results: A total of 579 infertile women are included. The mean serum 25-OH vitamin D concentration was 16.28 +/- 11.58 (range, 1-79.5) ng/mL; 220 patients were severely vitamin D deficient, 192 were vitamin D deficient, 95 were vitamin D insufficient, and only 72 patients were vitamin D sufficient. Vitamin D deficiency was more frequent in the younger patients (< 35 years) than in older patients (> 35 years) and the difference was statistically significant. Conclusion: This study showed that among persons presenting with infertility, more than three quarters of patients younger than 35 years and more than half of patients older than 35 years are vitamin D deficient and need treatment with vitamin D.