Background Morphology discrimination (MD) in implantable cardioverter-defibrillators (ICDs) is based on the comparison of the ventricular etectrogram during tachycardia with a stored reference template obtained during baseline rhythm. However, the effect of heart rate on the template match percentage during supraventricular tachyarrhythmias (SVT) is not known. The purpose of this study was to evaluate the performance of the template match percentage during SVT at different heart rates. Methods and results Stored electrograms of 868 tachyarrhythmias from 88 patients with a dualchamber ICD (St Jude Medical, USA) were analysed by the investigators. The effect of heart rate on template match percentage was estimated by regression analysis. For performance measures, data were corrected for multiple episodes in a patient by using the generalized estimating equation method. The mean template match percentage was 86.6 +/- 22.2% (median 100%) for SVT episodes. No significant differences in template match percentage between fast [ventricular cycle length (CL) 300-350 ms] and slow (ventricular CL >400 ms) SVTs were observed (85.4 +/- 27.0 vs. 87.1 +/- 19.7%). Using nominal settings, MD alone provided sensitivity and specificity of 70.2% and 89.4% overall, respectively. Morphology discrimination in conjunction with rate branch analysis, sudden onset, and stability yielded sensitivity and specificity of 98.5% and 91.2%, respectively. Conclusion Morphology discrimination has a consistently high template match percentage during SVFs, which is independent of ventricular CL. The consistent high match percentage results in high specificity for arrhythmia discrimination.