Importance: Vitamin D deficiency increases the incidence of respiratory virus infections. More than 1 billion people worldwide are vitamin D deficient. If vitamin D deficiency is associated to incidence or severity of SARS-CoV-2 infection, a global call could be made for vitamin D supplementation to mitigate the pandemic. Objective: to determine if lower serum 25-hydroxyvitamin D (25(OH)D) levels are correlated to the risk for COVID-19 and its severity as measured by CT Design: single-center observational study Setting: AZ Delta general hospital Participants: 186 consecutive patients with PCR-confirmed SARS-CoV-2 infection hospitalized for COVID-19 from March 1, 2020 to April 7, 2020 Main outcome and measures: comparative analysis of 25(OH)D levels in patients hospitalized for COVID-19 at various radiological stages and a season/age/sex-matched diseased control population Results: we report on 186 SARS-CoV-2 infected patients requiring hospitalization for severe COVID-19: 109 males (median age 68 years, IQR 53-79 years) and 77 females (median age 71 years, IQR 65-74 years). At admission patients were screened by CT to determine temporal changes of COVID-19 lung disease and classified as stage 1 (ground glass opacities), 2 (crazy paving pattern) and 3 (consolidation). At intake, 25(OH)D levels were measured and compared to a season-matched population of 2717 diseased controls, consisting of 999 males (median age 69 years, IQR 53-81 years) and 1718 females (median age 68 years, IQR 43-83 years). Male and female COVID-19 patients combined showed lower median 25(OH)D than controls (18.6 ng/mL, IQR 12.6-25.3, versus 21.5 ng/mL, IQR 13.9-30.8; P=0.0016) and a higher fraction of vitamin D deficiency (58.6% versus 45.2%, P=0.0005). A strong sexual dimorphism was found: female patients had comparable vitamin D status as control females. Male COVID-19 patients, however, showed markedly higher percentage of vitamin D deficiency than controls (67.0% versus 49.2%, P=0.0006) and this effect was more pronounced with advanced radiological stage ranging from 55.2% in stage 1 to 74% in stage 3. Conclusions and relevance: vitamin D deficiency is a possible risk factor for severe SARS-CoV-2 infection in males. Vitamin D supplementation might be an inexpensive, accessible and safe mitigation for the SARS-CoV-2 pandemic.