BACKGROUND The course of COVID-19 disease is associated with immune deregulation and excessive release of pro-inflammatory cytokines. Vitamin D has an immunomodulatory effect. We aimed to assess the possible correlation between the incidence and severity of SARS-CoV-2 infection and serum vitamin D concentration. MATERIAL AND METHODS A total of 505 successive patients admitted to a COVID-19-dedicated hospital were included in the retrospective analysis. Serum 25-hydroxyvitamin D (25-OHD) levels and SARS-CoV-2 RT-PCR throat swab test results were determined for each patient. The course of COVID-19 was assessed on the basis of the serum Vitamin Modified Early Warning Score (MEWS), which includes respiratory rate, systolic blood pressure, heart rate, temperature, and state of consciousness), as well as number of days spent in the intensive care unit (ICU) and need for oxygen therapy. RESULTS There was no difference in 25-OHD concentration between COVID-19-confirmed and negative results of the PCR tests. No correlation was found between serum 25-OHD in the COVID(+) group and the need for and time spend in the ICU, as well as the MEWS score. Multivariate analyses showed a positive correlation between need for oxygen therapy and lower 25-OHD concentration, as well as older age (P<0.001) and similar positive correlation between need for ventilation therapy with lower 25-OHD concentration, as well as older age (P=0.005). CONCLUSIONS Our findings do not support a potential link between vitamin D concentrations and the incidence of COVID-19, but low vitamin D serum level in COVID-19 patients might worsen the course of the disease and increase the need for oxygen supplementation or ventilation therapy.