A system and method for discriminating cardiac rhythms occurring in an antegrade direction from cardiac rhythms occurring in a retrograde direction. Atrial and ventricular contractions are sensed, from which atrial and ventricular cycle lengths are determined. Ventricular contractions are also analyzed to determine the occurrence of a tachycardia episode that has a one-to-one association of atrial contractions to ventricular contractions. During a tachycardia episode having a one-to-one association of atrial contractions to ventricular contractions, the atrial cycle lengths are paired with the ventricular cycle lengths, where for each of the atrial cycle lengths the atrial cycle length is paired with at least one ventricular cycle length started before the first atrial contraction of each of the atrial cycle lengths and paired with at least one ventricular cycle length started after the first atrial contraction of each of the atrial cycle lengths. A retrograde correlation coefficient is then determined for the atrial cycle lengths paired with the ventricular cycle lengths started before the first atrial contraction, and an antegrade correlation coefficient is determined for the atrial cycle lengths paired with the ventricular cycle lengths started after the first atrial contraction. The tachycardiac episode is then classified based on a comparison of the antegrade correlation coefficient and the retrograde correlation coefficient.