OBJECTIVE This work aimed to study the delayed electrocardiographic changes, including Q-T interval, corrected Q-T dispersion, and heart rate variability (HRV) 3 years after percutaneous transluminal septal myocardial ablation (PTSMA), in symptomatic patients with hypertrophic obstructive cardiomyopathy (HOCM). METHODS In 26 patients (11 women, 15 men; average age, 37.4 +/- 11.2 years) with symptomatic and medically refractory HOCM, 1.4 +/- 0.5 septal branches were occluded with an injection of 3.8 +/- 1.3 mL of alcohol (95%) to ablate the hypertrophied interventricular septum. Baseline and 3 days and 3 years postprocedure 24-hour Holter electrocardiographic findings were determined. RESULTS One patient developed complete atrioventricular block requiring permanent pacing. The PR interval was significantly prolonged 3 days after ablation, but recovered 3 years postprocedure. Three days after the procedure, all patients developed right bundle branch block, which was present in 24 patients after 3 years. The QRS duration was significantly prolonged 3 days after ablation and during 3 years of follow-up. There was significant and persistent prolongation of QT interval and transient prolongation of corrected QT dispersion 3 days after ablation and returned to preablation values 3 years postprocedure, but JT interval and corrected JT dispersion were not significantly changed after PTSMA. Heart rate variability data (time domain and frequency domain) 3 days and 3 years after PTSMA, including low frequency, high frequency, root mean squared successive difference interval, and the percent of sinus cycles differing from the preceding cycle by more than 50 milliseconds, significantly increased compared to that before the procedure. Low frequency/high frequency, SD of all normal-to-normal intervals, and SD of 5-minute average normal-to-normal intervals were not significantly changed after PTSMA. CONCLUSIONS Percutaneous transluminal septal myocardial ablation for HOCM induces significant delayed electrocardiographic changes in most patients. The changes include QRS prolongation, new right bundle branch block, persistent QT prolongation, transient QT dispersion and PR prolongation, and changes in HRV data. Electrocardiographic long-term follow-up of a larger series of patients is required to determine the conclusive therapeutic significance.