A total of 664 high school students having >or=1 episode of syncope and 560 students without syncope (controls) underwent 12-lead ECG. A significant QT prolongation was defined as corrected QT (QTc)>470 ms in male and >480 ms in female. A diagnosis of LQTS was made by Schwartz's score. Most students had only 1 episode of syncope (66.1%), 2 (16.9%), 3 (11.6%), and >or=4 (5.4%). Most syncopal ... [Show full abstract] episodes were related to non-exertional events with only 11.4% related to exertion. The QTc distribution of the study group was almost identical to controls with mean QTc of 405.1+/-23.2 vs 402.5+/-23.4 ms, respectively. Only one male (0.15%) in the study group had significant QTc prolongation. None met the criteria for diagnosis of LQTS (score>or=4). In conclusion, the prevalence of LQTS in adolescents having syncope seems to be low in this cohort study.