The value of the 12-lead electrocardiogram for distinguishing atrioventricular (AV) nodal reciprocating tachycardia from circus movement AV tachycardia utilizing a retrograde accessory pathway was studied in 100 patients with narrow QRS complex tachycardia. Intracardiac electrograms showed AV nodal reciprocating tachycardia in 40 patients and circus movement AV tachycardia in 60. The 12-lead electrocardiograms recorded during tachycardia were randomly sorted and reviewed by 4 experienced cardiac electrophysiologists who were blinded to the diagnosis associated with each tracing, the relative proportion of each arrhythmia and the hypotheses to be tested. Each reviewer was asked to indicate the location of the P wave relative to the QRS complex, electrical axis of the P wave in the frontal and horizontal planes and presence or absence of QRS alternation, and to interpret the most likely mechanism. The performance of published electrocardiographic criteria to differentiate AV nodal reciprocating tachycardia from circus movement AV tachycardia was evaluated. The overall accuracy of the reviewers' interpretations was 75%, similar to the accuracy of the predefined criteria when applied by these observers (71% correct, difference not significant). Interobserver agreement of reviewer interpretations was 76% and the intraobserver agreement was 78%. Features associated with circus movement AV tachycardia by univariable analysis were P waves after the QRS complex, faster tachycardia rates and QRS alternation. Multivariable analysis showed that only the location of the P wave relative to the QRS complex was independently associated with the mechanism of tachycardia (p = 0.002). QRS alternation was found by multivariate analysis to be associated with the rate but not the mechanism of the tachycardia.