We report the case of a 59-year-old man with a dilated nonischemic cardiomyopathy who had chronic rapid atrial fibrillation despite several therapies. Radiofrequency modification of the atrioventricular (AV) junction slowed the mean ventricular rate from 120 beats per minute (bpm) to 60 bpm. Five hours after the procedure and during the following 1 week, despite ventricular pacing at 90 bpm, the patient developed nonsustained or sustained polymorphic ventricular tachycardias. Finally, pacing at 90 bpm was successfully used in this patient. In conclusion, patients who have undergone modification of AV conduction may be at high risk of life-threatening ventricular arrhythmias in the first week following the procedure.