Background: Multiple blood markers were correlated with poor clinical results in severely ill COVID-19 patients. Published reports correlated C-Reactive Protein (CRP), Neutrophil-to-Lymphocyte Ratio (NLR), D-dimer, serum ferritin, and vitamin D levels, with adverse outcomes and an enhanced mortality rate in vitally ill COVID-19 patients; yet, evidence is not enough to guide management for the patients using the predictive values of the above mentioned markers. Objectives: to assess the impact of vitamin D on the outcomes of COVID-19 patients who were extremely ill.  Methods: We conducted prospective research on 40 COVID-19 patients who were critically sick and hospitalized to the intensive care unit (ICU) of a chest isolation hospital in Beni Suef, Egypt (COVID intensity score ≥  8). Vitamin D concentrations in blood were determined at admission and 48 hours later. Results: There were 40 patients total, with an average age of 55.6 ±9.9 years and a non-statistical significance between both sexes. With a median duration of 15 days, mechanical breathing was necessary for 62.5% of the patients (invasive and non-invasive). There was a clinical correlation between death and male sex, the existence of diabetes mellitus, bilateral pulmonary infiltration, and heart failure, despite these correlations being statistically insignificant. After admission, serum vitamin D levels did not significantly influence mortality prediction; however, 48 hours later, vitamin D significantly affected mortality prediction, with Area under curve (0,886) and sensitivity (81,8), specificity (83,3), PPV (85,7), and NPV (78,9) at a cut-off ≤ 18. Conclusions: vitamin D useful as a predictor for mortality. Many demographic and clinical factors must be correlated with , when interpretating this biomarker .