The role of 25-OH-vitamin D in the assessment of coronavirus disease 19 (COVID-19) has not been investigated. We sought to investigate the prevalence of 25-OH-vitamin D deficiency among COVID-19 patients, and to determine the associations between 25-OH-vitamin D status and the severity of the disease. We have conducted a retrospective observational study of COVID-19 patients admitted to the University of Verona Hospital Trust. Demographic, clinical and biochemical parameters were collected at hospital admission, and serum 25-OH-vitamin D levels were measured. The following outcomes were assessed: arterial partial oxygen pressure (PaO2); C-reactive protein (CRP); length of hospitalization; requirement of oxygen therapy; non-invasive ventilation (NIV); mechanical ventilation; and death. Among 61 patients enrolled, 72.1% was 25-OH-vitamin D deficient (<20 ng/mL) and 57.4% had 25-OHvitamin D <15 ng/mL. Patients with arterial PaO2 <60 mmHg had significantly lower mean 25-OH-vitamin D levels compared to patients with PaO2 ≥60 mmHg (13.3 ng/mL vs 20.4 ng/mL respectively, p=0.03). Vitamin D deficiency was associated with 3-fold higher risk of having arterial pO2 <60 mmHg. 25-OH-vitamin D deficiency was associated with increased CRP and dyspnea. 25-OH-vitamin D deficiency was associated with more severe systemic inflammatory response and respiratory failure in COVID-19 patients.