Heart Rhythm 2004, the 25th Annual Scientific Sessions of the Heart Rhythm Society (formerly the North American Society of Pacing and Electrophysiology), met in San Francisco in May 2004. The meeting is the world's premier forum for the presentation of research and the exchange of state-of-the-art information in cardiac electrophysiology and pacing. Major new research findings were presented on the value of implantable cardioverter-defibrillator therapy, noninvasive methods for risk stratification, treatment of vasovagal syncope, radiofrequency ablation for atrial fibrillation, resynchronization therapy for heart failure, and new insights from basic science into the mechanisms of cardiac arrhythmias.