Background: Vitamin D is a group of fat-soluble corticosteroid responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and multiple other biological effects. In humans, the most important compounds in this group are vitamin D3 [also known as cholecalciferol] and vitamin D2 [ergocalciferol].Aim of the work: The aim of this thesis is to assess Vitamin D3 [VD3] level in patients with sino-nasal polypi in comparison with normal individuals.Patients and Methods: This prospective comparative [case-control] study conducted by the Department of ENT, Al-Azhar University Hospitals in the period from March 2019 to September 2019 and included 40 individuals who were selected by inclusion and exclusion criteria, both genders, all patients had been counseled about the nature of the study and informed written consent has been obtained.Results: Serum level of VD3 in patients with chronic rhinosinusitis with sinonasal polyps [CRSwNP] and allergic fungal rhinosinusitis [AFRS] was significantly lower than that of control subjects. Vitamin D levels ranged from 8.74 to 39 ng/ml, and there was statistically significant decrease of serum vitamin D levels in patient group when compared to control group [16.20±4.29 vs 30.56±4.83 respectively]. In addition, there was significant increase of patients with deficient and insufficient vitamin D levels in patient group when compared to control group.Conclusion: VD3 may constitute an inexpensive prophylactic and cost effective option in the therapeutic armamentarium in reducing inflammation either by itself or as a synergistic agent to traditional agents in patients suffering from sino-nasal polypi.