Radiofrequency catheter ablation of accessory bypass tracts associated with the Wolff-Parkinson-White (WPW) syndrome has become an accepted and widespread therapy. When bypass tracts are located in the free wall of the left ventricle, complete atrioventricular (AV) block is an unusual complication. Two cases of symptomatic WPW syndrome with transient complete atrioventricular block during catheter ablation are described. The first case was a 14-year-old female with an accessory pathway located in the left posterior wall, and the second was a 72-year-old female with an accessory pathway located in the left lateral wall. Radiofrequency energy application resulted in transient complete AV block with escape rhythm. In the first case, AV conduction with left bundle branch block resumed the next day, whereas in the second case, AV conduction soon resumed with prolongation of atrio-His (AH) interval and no evidence of pre-excitation. This phenomenon could have been due to either trauma to the AV node during catheter entry into the left ventricle or compression of the AV node with a catheter shaft during ablation because both patients' hearts were comparatively small.