<br><b>Background</b><br>Vitamin D (VD) and its metabolites reduce the incidence of various cancers, including head and neck squamous-cell carcinoma (HNSCC) by inhibiting tumor angiogenesis, stimulating mutual adherence of cells, and enhancing intercellular communication, thereby strengthening the inhibition of cellular proliferation. Epidemiologic and case–control studies have demonstrated that low VD levels are associated with HNSCC risk. Some studies showed that patients with HNSCC who received VD at the time of diagnosis had diminished peripheral blood and intratumoral levels of immunosuppressive CD34 + cells and increased levels of mature dendritic cells. In addition, they showed the quantitative increase of the immune cells within the HNSCC tissue following VD treatment.<br><b>Aim</b><br>To investigate VD levels in head-cancer and neck-cancer patients as a step to further elucidate its association with head and neck cancer development, progression, and outcome.<br><b>Participants and methods</b><br>This study showed that VD deficiency is prominent in patients with head and neck cancer than in controls. Although it may increase patients' risk of therapy-related morbidity and poor outcome, it represents an inexpensive prophylactic and cost-effective option in the therapeutic armamentarium as a synergistic agent to traditional treatment strategies.<br><b>Results</b><br>The mean VD level in group A was 7.4 ± 2 ng/ml and for group B was 21 ± 6 ng/ml (<i>P</i>=0.043), indicating a significant decrease of VD in group A than in group B. Also, VD-deficiency status was significantly correlated with lower hemoglobin levels and with hepatitis C virus seropositivity in the studied cases.<br><b>Conclusion</b><br>This study showed that VD deficiency is prominent in patients with head and neck cancer than in controls. Although it may increase patients' risk of therapy-related morbidity and poor outcome, it represents an inexpensive prophylactic and cost-effective option in the therapeutic armamentarium as a synergistic agent to traditional treatment strategies.<br>