Data were collected prospectively from sixteen clinical centers in the United States and Europe on attempted catheter ablation of the atrioventricular junction, accessory atrioventricular pathways and ventricular tachycardia. Each of the three ablation techniques was studied as a separate subset and the two methods of energy delivery, direct current or radiofrequency energy, were compared within each subset. A standardized protocol was used for direct current ablation and the results of a variety of protocols for the other two procedures and for radiofrequency energy were observed. Patients were evaluated quarterly in the first post-procedure year, and biannually for the subsequent duration of the study. Statistical analyses were performed to determine the distribution of specific patient characteristics such as age, sex, clinical status, underlying heart disease, and a variety of procedural categories including energy level, mapping data, and catheter position. A determination was made of the frequency of successful outcomes including cessation of arrhythmias, production of third degree atrioventricular block, and disappearance of symptoms and of adverse effects, including procedure-related complications and death.