OBJECTIVE: In our study, it was purposed to research the distribution of vitamin D according to age, sex, and seasons of the patients diagnosed with obesity, cancer, and chronic renal failure who applied to Inonu University Turgut Ozal Medical Center between 2018-2021. MATERIALS AND METHODS: In our descriptive study, vitamin D measurements from 01.01.2018-01.01.2021 users who visited Inonu University Turgut Ozal Medical Center and were diagnosed with obesity, cancer, and chronic renal failure were scanned in their files and assessed retrospectively. The patients were grouped according to their sex, age, and seasons in which the samples were taken. 25(OH)D levels were analyzed by LC-MS/MS method. The patients were classified as deficient, inadequate, and normal in terms of vitamin D level RESULTS: Vitamin D deficiency was monitored in 54.9% (n = 981) of 1787 cancer patients admitted to our hospital, 63.3% (n = 558) of 882 chronic renal failure patients, and 65.8% (n = 1903) of 2894 obese patients. CONCLUSION: Of the 5563 obesity, cancer, and chronic renal failure patients who applied to our hospital; vitamin D deficiency was found in 61.9% (n = 3442), vitamin D deficiency was found in 23.1% (n = 1284), while vitamin D level was found to be normal in only 15% (n = 837). In terms of vitamin D deficiency, the highest deficiency was seen in obese patients with a rate of 57%. In our study, it was observed that vitamin D levels vary depending on the diseases, seasons, and age groups examined.