Objective: Toevaluate the relationship between the vitamin D levels of patients at admission and coronavirus disease 2019 (COVID-19)prognosis and mortality. Materials and methods: This study was conducted between July 2020 and January 2021 with 243 patients whose severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)reverse transcription-polymerase chain reaction (RT-PCR) test was positive and who were being monitored and treated for COVID-19 diagnosis in Selçuk University Medical School Hospital. Based on thoracic computed tomography results, the patients were first divided into 2 groups with respect to presence of typical involvement for COVID-19. Then, those who had involvement in their thoracic CT scans were further divided into 3 groups based on the extent of their lesions. Results: No correlation was found between the vitamin D values of the patients and the presence or level of involvement in their thoracic tomography scans. The vitamin D level was above 25 ng/mL in 30 of the 243 patients, while it was below that in 213 patients. Based on the logistic regression analysis, in which a vitamin D level<25 ng/mL was accepted as vitamin D deficiency, the risk of mortality was 2.88 (1.037-8.015, P=0.042) times greaterin those who had a vitamin D deficiency than in those who did not. Conclusion: While vitamin D deficiency did not have any significant effect on prognosis in thisstudy, it was found to increase mortality. Despite this, there is not sufficient evidence regarding the relationship between vitamin D levels and the severity of COVID-19 and death rates. There is a need for randomized controlled studies and large-scale cohort studies to test this hypothesis.