Multi-center, prospective, five-year study. The collection of these data was intended to provide the following information: 1) the incidence of serious cardiac damage as a result of ablation; 2) the incidence and time course of recurrence after initially successful ablation; and 3) the incidence of proarrhythmia following ablation. A total of 480 pediatric patients were enrolled prospectively and evaluated both before ablation of supraventricular tachycardia and at intervals following ablation with clinical history and examination, electrocardiogram, 24-hour Holter monitor, and echocardiogram, with non-invasive studies read by outside consultants. In addition, a complete Registry of pediatric patients undergoing ablation at the participating centers was established to allow comparisons with the study group and to provide population estimates of success and complication rates.