Context. Through its immunological functions, vitamin D attenuates inflammatory responses to respiratory viruses. Vitamin D deficiency might be a highly prevalent risk factor for severe SARS-CoV-2 infections. Objective. To investigate the level of vitamin D deficiency in West Flanders, Belgium and its correlation to severity of COVID-19 as staged by CT Design. Retrospective observational study Setting. Central network hospital Participants. 186 SARS-CoV-2-infected patients hospitalized from March 1, 2020 to April 7, 2020 Main outcome measure. Analysis of 25(OH)D in COVID-19 patients versus season/age/sex-matched diseased controls Results. The rate of vitamin D deficiency (25(OH)D<20 ng/mL) in West Flanders varies with age, sex and season but is overall very high (39.9%) based on analysis of 16274 control samples. We measured 25(OH)D levels in 186 COVID-19 patients (109 males (median age 68 years, IQR 53-79) and 77 females (median age 71 years, IQR 65-74)) and 2717 age/season-matched controls (999 males (median age 69 years, IQR 53-81) and 1718 females (median age 68 years, IQR 43-83)). COVID-19 patients showed lower median 25(OH)D (18.6 ng/mL, IQR 12.6-25.3, versus 21.5 ng/mL, IQR 13.9-30.8; P=0.0016) and higher vitamin D deficiency rates (58.6% versus 45.2%, P=0.0005). Surprisingly, this difference was restricted to male COVID-19 patients who had markedly higher deficiency rates than male controls (67.0% versus 49.2%, P=0.0006) that increased with advancing radiological stage and were not confounded vitamin D-impacted comorbidities. Conclusions: vitamin D deficiency is a prevalent risk factor for severe COVID-19. Vitamin D supplementation might be an inexpensive and safe mitigation for the SARS-CoV-2 pandemic.