An implantable cardioverter/defibrillator system provided with method and apparatus for discrimination between monomorphic arrhythmias, e.g. ventricular tachycardia from polymorphic arrhythmias, e.g. ventricular fibrillation. A fiducial point of each successive QRS complex is detected prompting the storage of sampled and digitized waveform data within a timing window bridging the point in time of fiducial point detection. Stored sets of such sampled wave shape data are compared data point to data point resulting in a sampled morphology index value for each compared set. The magnitude of the sampled morphology index value or a series such index values are analyzed to determine the presence of a single or a progression of beat-to-beat waveform changes indicative of a polymorphic single transition or progression of QRS complexes from monomorphic to polymorphic waveforms indicative of an arrhythmia that should be treated with aggressive cardioversion/defibrillation therapies. The system is preferably provided with a closely spaced and widely spaced pairs of electrodes for sensing each QRS complex of the patient's electrocardiogram. The closely spaced electrode pair is coupled to sense detect circuitry for identifying a predetermined fiducial point in the electrical signal associated with a ventricular depolarization and to counting and comparison circuitry for developing rate and onset data. The widely spaced pair of electrodes is coupled to sense and digitizing circuitry for developing the sampled waveform amplitude data from which the morphology index values are derived.