© 2020, Nobelmedicus. All rights reserved.Objective: We aimed to investigate the vitamin D levels of patients aged 18 years and older who applied to the family medicine outpatient clinic according to sex, years, seasons and months. Material and Method: This descriptive study was carried out retrospectively based on the medical records of patients whose vitamin D levels were measured between 02.01.2015 and 31.12.2018. The patients were grouped according to year, season and month in which vitamin D level was measured. Data of 4902 patients were analyzed. Results: The median age of the patients was 28.0 (IQR: 21.0-42.0) years. 78.8% of the patients were female. The median vitamin D level of the patients was 13.4 ng/ml. Vitamin D deficiency was found in 76.4% and vitamin D insufficiency was found in 17.5% of the patients. Vitamin D was normal in only 6.1% of the patients. Vitamin D levels were significantly lower in women than men (p<0.001). Vitamin D deficiency was the most common in the spring (84.5%, p<0.001). There was a significant difference between vitamin D levels by months (p<0.001). The month with the highest vitamin D deficiency was march with 88.7%. Conclusion: Vitamin D deficiency rates were found to be quite high in our study. People should be informed about how to benefit from sunlight, how often and how vitamin D-rich foods should be consumed. High risk individuals should be provided the recommended doses of vitamin D in order to prevent and treat vitamin D deficiency.