Sudden intense physical activity is a trigger for ventricular arrhythmias in long QT syndrome (LQTS), and beta-blockers (B) reduce the risk of ventricular arrhythmias in LQTS. We compared the effect of graded (gradual intensity) versus burst (sudden intensity) exercise on QT-rate adaptation and T wave complexity in LQTS + B (n = 21), LQTS - B (n = 5), and normal controls (n = 20). Graded exercise consisted of symptom-limited bicycle ergometry (30 W, increment 20 W/min). Burst exercise involved a fixed 200 W load for 1.0 minute. ECGs were digitally recorded every 10 seconds during exercise and a 10 minute recovery period. QT-rate adaptation was quantified using the slope of the QT cycle length relationship fit to a quadratic function. Principle component analysis (PCA) was used to quantify T wave complexity. The QT-rate slope was two-fold greater with burst exercise than graded exercise for LQTS + B (-82E +/- 40E vs -36E +/- 40E, P = 0.0016), LQTS - B (-85E +/- 60E vs -30E +/- 50E, P = 0.011) and controls (-100E +/- 60E vs -48E +/- 100E, P = 0.0011) (E = x10(-5)). For each exercise protocol, there was no difference in QT-rate slope between the three groups. In contrast, the QT-rate slope during the 10 minute recovery period was similar between the burst and graded protocol for LQTS + B (25E +/- 40E vs 30E +/- 50E), LQTS - B (81E +/- 80E vs 85E +/- 70E) and controls (90E +/- 80E vs 82E +/- 80E). The coefficient of variability of PCA (T wave complexity variability) during burst exercise was greater than that during graded exercise for LQTS + B (41 +/- 15 vs 30 +/- 10, P = 0.017), LQTS - B (47 +/- 25 vs 26 +/- 4, P = 0.016), and control (46 +/- 14 vs 33 +/- 19, P = 0.012). For each exercise protocol, no difference in T wave complexity variability was seen between the three groups. In conclusion, QT heart rate slope and T wave complexity variability are greater during sudden intense exercise than graded exercise in LQTS patients (on and off beta-blockers) and normal subjects, with similar findings among the three groups of patients.