BACKGROUND No prior studies have systematically investigated the diagnostic value of cycle length (CL) variability in differentiating the mechanism of paroxysmal supraventricular tachycardia (PSVT). RESULTS We studied 173 consecutive patients with PSVT; 86 typical atrioventricular nodal reentrant tachycardia (AVNRT), 11 atypical AVNRT, 47 orthodromic reciprocating tachycardia (ORT), and 29 with atrial tachycardia (AT). Two consecutive atrial cycles that displayed the most CL variability were selected for analysis. One hundred and twenty-six patients (73%) had > or = 15 msec variability in tachycardia CL. The change in atrial CL predicted the change in subsequent ventricular CL in six of eight patients (75%) with atypical AVNRT, 18 of 21 patients (86%) with AT, in none of 66 patients with typical AVNRT, and in 32 patients with ORT. The change in atrial CL was predicted by the change in preceding ventricular CL in 55 of 66 patients (83%) with typical AVNRT, no patient with atypical AVNRT, 27 of 31 patients (87%) with ORT, and one of 21 patients (5%) with AT. The sensitivity, specificity, and positive and negative predictive values of a change in atrial CL predicting the change in ventricular CL for AT or atypical AVNRT were 83%, 100%, 100%, and 95%, respectively. The corresponding values for the change in atrial CL being predicted by the change in the preceding ventricular CL for typical AVNRT or ORT were 85%, 97%, 99%, and 65%, respectively. CONCLUSIONS Tachycardia CL variability > or = 15 msec is common in PSVT. A change in atrial CL that predicts the change in subsequent ventricular CL strongly favors AT or atypical AVNRT. A change in atrial CL that is predicted by the change in the preceding ventricular CL favors typical AVNRT or ORT.