The primary aim of the present study was to investigate the short-term effects of surgical left cardiac sympathetic denervation (LCSD) on the QT interval and heart rate in patients with congenital long QT syndrome (LQTS). Left cardiac sympathetic denervation was performed in five LQTS patients who had a history of syncope. The patients' 12-lead and 24-h Holter monitoring ECG was recorded 24'h before and 24'h after LCSD. Treadmill exercise tests were also performed before and 6 days after surgery to assess changes in heart rate and the QT interval after surgery. Left cardiac sympathetic denervation was successful in all patients. The mean value of the corrected QT interval (QTc) in the five patients decreased from 0.59 ± 0.05 to 0.48 ± 0.04's (P = 0.006) immediately after the procedure and remained short (0.47 ± 0.04, P < 0.05) after a 21-month follow-up. The mean value of QTc on the 24-h Holter monitoring ECG also decreased in all patients (0.67 ± 0.07 vs 0.60 ± 0.05's, P < 0.01). The mean, maximum, and minimum heart rate on the 24-h ECG remained unchanged (P > 0.05). The maximum heart rate during the exercise tests decreased from 162 ± 4 beats/min before surgery to 129 ± 10 beats/min (P < 0.01). The exercise-induced increase in QTc remained unchanged after the surgery (P > 0.05). Although four of the five patients were syncope-free until 21 months postoperatively, the remaining patient had a recurrence of syncope, requiring an increased dose of ÃŸ blocker. These findings indicate that LCSD shortens QTc and diminishes the exercise-induced increase in heart rate whereas the resting heart rate and exercise-induced increase in QTc remain unchanged. These results may have implications for the effectiveness and limitations of LCSD.