An implantable cardiac stimulation device classifies ventricular tachyarrhythmias as either a ventricular tachycardia or ventricular fibrillation and applies appropriate therapy. The device include sensing circuits that provide right and left ventricular electrogram signals, a detector that detects an accelerated ventricular arrhythmia, and a classifying circuit that measures relative correspondence between the right and left ventricular electrogram signals to classify a detected accelerated ventricular arrhythmia as either ventricular tachycardia or ventricular fibrillation. A therapy circuit is responsive to the classifying circuit and applies appropriate therapy to the heart based upon the tachyarrhythmia classification.