Measurements of the Q-T interval are less reliable in children than in adults. Identification of superior diagnostic tools is warranted. This study aimed to investigate whether a vectorcardiogram (VCG) recorded from three orthogonal leads (X, Y, Z) according to Frank is superior to a 12-lead electrocardiogram (ECG) in providing a correct long Q-T syndrome (LQTS) diagnosis in children. This LQTS group consisted of 35 genetically confirmed carriers of mutations in the KCNQ1 (n = 29) and KCNH2 (n = 6) genes. The control group consisted of 35 age- and gender-matched healthy children. The mean age was 7 years in the LQTS group and 6.7 years in the control group (range, 0.5–16 years). The corrected Q-T interval (QT_c) was measured manually (QT_man) by one author (A.W.). The 12-lead ECG automatic measurements (QT_ECG) and interpretation (QT_Interpret) of QT_c were performed with the Mac5000 (GE Medical System), and the VCG automatic measurements (QT_VCG) were performed with the Mida1000, CoroNet (Ortivus AB, Sweden). By either method, a QT_c longer than 440 ms was considered prolonged and indicative of LQTS. Of the 35 children with genetically confirmed LQTS, 30 (86 %) received a correct diagnosis using QT_VCG, 29 (82 %) using QT_man, 24 (69 %) using QT_ECG, and 17 (49 %) using QT_Interpret. Specificity was 0.80 for QT_VCG, 0.83 for QT_man, 0.77 for QT_ECG, and 0.83 for QT_Interpret. The VCG automatic measurement of QT_c seems to be a better predictor of LQTS than automatic measurement and interpretation of 12-lead ECG.