BACKGROUND: Vitamin D deficiency (VDD) is a known risk factor for tuberculous infection. We investigated if VDD is a risk factor for tuberculous infection among the household contacts (HHCs) of patients with tuberculosis (TB) in Mongolia.MATERIALS and METHOD: All HHCs of TB patients diagnosed in Khan-Uul District, Mongolia, were enrolled. The serum level of 25-hydroxyvitamin D [25(OH)D] was detected and TB infection determined using QuantiFERON-TB Gold Plus (QFT-Plus). A tuberculin skin test (TST) reading >10 mm was considered to be positive. Epidemiological and bacteriological data were collected from routine surveillance of the National Tuberculosis Programme.RESULTS: Among study participants, 48.2% (135/285) were QFT-Plus-positive. Of QFT-positive HHCs, 77.0% (104/135) were TST-positive and the overall concordance of tests was low (κ 0.374, P < 0.001). A low serum level of 25(OH)D was an independent predictor for QFT-Plus positivity (P < 0.001). CD8+ T-cell stimulation measured by QFT-Plus had borderline association with the serum level of 25(OH)D (P = 0.089).CONCLUSION: We showed a high rate of TB infection among HHCs in Mongolia. QFT-Plus could decrease the number of people requiring TB preventive treatment, in addition to aiding detection of new TB infection. A low serum level of vitamin D was an independent predictor of TB infection, but not a predictor of stimulation of CD8+ T cells.