Deep neck infection (DNI) refers to infections in spaces created by superficial and deep cervical fascia around the muscles and organs in the neck. Vitamin D is highly important for an effective immune system. Vitamin D receptors (VDR) have been identified in immune system cells, and particularly in T and B lymphocytes, macrophages, and dendritic cells. Vitamin D deficiency is thought to result in impaired immune response, decreased leukocyte chemotaxis, and an increased disposition to infection. The purpose of this study was to investigate whether vitamin D deficiency is an underlying occult factor in the development of DNI. Sixty-five patients aged 6 to 90, diagnosed with DNI, and 70 healthy age- and sex-compatible cases were included in the study. Serum levels of calcium, phosphorus, parathyroid hormone, and 25-hydroxy vitamin D (25(OH)D) were determined in each case. 25-hydroxy vitamin D levels above 20 ng/mL were regarded as normal, 12 to 20 ng/mL as insufficient, 5 to 12 ng/mL as deficient, and less than 5 ng/mL as severely deficient. Mean serum 25(OH)D levels were 10.4 (6.2) ng/mL in the patient group and 15.5 (6.4) ng/mL in the control group (P< .01). This difference was statistically significant (P< .01). Vitamin D was within normal limits in 9.2% (n = 6) of cases in the study group, insufficient in 29.2% (n = 19), deficient in 35.3% (n = 23), and severely deficient in 26.2% (n = 17). The equivalent values in the control group were 21.4% (n = 15), 48.5% (n = 34), 30% (n = 21), and 0% (n = 0). Serum 25(OH)D levels were significantly lower in patients with DNI compared to the healthy cases; 25(OH)D levels may be a factor in the development of DNI.