There are still few studies that evaluated QuantiFERON TB-2G (QFT) response in children with tuberculosis (TB).We evaluated the performance of this assay in 38 children with active TB disease (including 17 children with bacteriologically confirmed TB disease) treated in 4 hospitals.Before starting anti-TB chemotherapy, 29 (82.9%) of 35 children with active TB disease were positive in the QFT test, suggesting that this assay might be very useful for diagnosing active TB disease also in children.However, negative results were found in 2 cases with smear-positive pulmonary TB.In these two cases, as negative QFT results and lower responses to non-specific mitogen, PHA, were observed, cellular immunity was suggested to be lowered.Thus, in the cases with lowered cellular immunity, negative QFT results should not be used to exclude active TB disease.