An implantable cardioverter/defibrillator (ICD) includes a tachyarrhythmia detection and classification system that classifies tachyarrhythmias based on a morphological analysis of arrhythmic waveforms and a template waveform. Correlation coefficients each computed between morphological features of an arrhythmic waveform and morphological features of the template waveform provide for the basis for classifying the tachyarrhythmia. In one embodiment, morphological features are collected from a sensed arrhythmic waveform, and temporally corresponding morphological features are extracted a stored template waveform. In one embodiment, a correlation analysis takes into account the uncertainty associated with the production of the template waveform by using a template band that includes confidence intervals. In one embodiment, a correlation analysis produces Mahalanobis distance-based correlation coefficients for use in the classification of the tachyarrhythmia. In one embodiment, the stability of correlation coefficients are analyzed to classify the tachyarrhythmia.