OBJECTIVE: To study the effect of sympathetic stimulation and increase in heart rate on the QT and QTc intervals. DESIGN: Prospective non-randomised study of eight consecutive patients. SETTING: Electrophysiology laboratory at a tertiary centre. PATIENTS: Eight patients aged 10-20 years (median 12.5) undergoing repeat electrophysiological study after previously successful catheter ablation (n = 6) or presumed supraventricular tachycardia (n = 2) with negative studies. INTERVENTIONS: Electrocardiograms were obtained (a) at baseline, (b) during atrial pacing at 450 ms cycle length, (c) during isoprenaline infusion at 0.025 microgram/kg/min, (d) adding atrial pacing (450 ms cycle length) to isoprenaline at 0.025 microgram/kg/min, and (e) isoprenaline at 0.05 microgram/kg/min. MAIN OUTCOME MEASURES: QT and QTc intervals at each of the above mentioned stages. RESULTS: The QT interval was reduced from a mean value of 350 ms to around 315-325 ms by each of the above manoeuvres. Correspondingly, the QTc increased from a mean of 407 ms to around 445-470 ms. Pacing was as effective as isoprenaline in shortening the QT interval and prolonging the QTc intervals. CONCLUSIONS: Heart rate directly influences QT and QTc intervals in children and adolescents. The QT is shortened, but QTc is prolonged. Hence, reliance on the QTc alone could lead to mistaken diagnosis of long QT syndrome.