Introduction: SARS-CoV-2 (COVID-19), which emerged in Wuhan, China in December 2019, infected more than six million people in a short time. In COVID-19, the relationship of many laboratory parameters to morbidity and mortality has been defined. In our study, we aimed to determine the relationship of serum vitamin D level to clinical course and prognosis. Materials And Methods: This study included 108 patients; 88 patients who stayed in Ataturk University and Erzurum City Hospital between March 24, 2020 and May 15, 2020, who were identified as COVID-19 by real-time PCR method from the nasopharyngeal swab and 20 asymptomatic voluntary medical personnel who tested negative for real-time PCR after routine check-up in our hospital.Result: In statistical analysis conducted between healthy control group and vitamin D levels of patients admitted due to COVID-19, it was observed that patients infected with COVID-19 had a lower level (p= 0.004). In 20 patients developing MAS, a lower level of vitamin D was observed (p= 0.004) compared to 68 patients who did not develop. In the comparison of vitamin D levels of the patients (n= 8) who developed exitus in their follow up due to COVID-19, it was observed that vitamin D levels were statistically significantly lower compared to the living (p= 0.009).Conclusions: Due to COVID-19, pandemic, long-running quarantines caused insufficient use of sunlight and worsening of vitamin D deficiency. We wanted to draw attention again with our study to vitamin D which can be responsible for the heavy clinical course of COVID-19 and whose replacement is easy to apply.