Abstract: Objective To investigate the relationship between QT dispersion and torsades de pointes (TDP) in idiopathic long QT syndrome (LQT1). Methods Seventy-nine carriers of the gene on the chromosome 11p15.5 were divided into 3 groups according to the severity and frequency of syncope episodes, namely the mild (0-4 episodes, n=59), moderate (5-100 episodes, n=14) and severe (with history of cardiac arrest, n=5). The differences in QT dispersion in the initial ECGs recorded were compared between the 3 groups, and the QT dispersion changes after chronic beta-blocker therapy in 19 moderate to severe cases under 35 years, along with the changes during exercise in 9 cases before and after beta-blocker therapy were evaluated and compared. Results QT dispersion was significantly different between the 3 groups (P<0.01), and after chronic beta-blocker therapy, QT dispersion decreased in 10 of the 13 cases (P<0.05). During the exercise test, the QT dispersion was positively correlated with the exercise load before beta-blocker treatment but inversely correlated after the treatment (P<0.01). Conclusions QT dispersion is a sensitive and reliable marker to predict TDP and beta blocker should be given to moderate or severe LQT1 cases under 35 years, especially in the asymptomatic cases at risk of TDP.