Risks associated with radiofrequency ablation (RFA) have been reported to be increased in children < or =15 kg. We sought to compare the safety and efficacy of RFA in children <15 kg with those between 15.1 and 20 kg. Clinical, electrophysiologic, and RFA data for all patients < or =20 kg who underwent RFA for supraventricular tachycardia between January 1994 and January 2003 were reviewed. Patients were divided into those < or =15 kg (group 1, n = 25) and those between 15.1 and 20 kg (group 2, n = 44). The two groups differed significantly in age and weight by design (group 1: mean weight, 11.9 +/- 3.0 kg; age, 2.8 +/- 1.9 years; group 2: weight, 18.0 +/- 1.5 kg; age, 5.1 +/- 1.1 years). There were no significant differences in other baseline characteristics except for incidence of structural heart disease (28% group 1 vs 7% group 2, p < 0.01). No significant differences in mechanism of tachycardia, arrhythmia cycle length, number of total and brief RFA applications, total RFA time, average and maximum RFA temperatures, total procedure duration, short-term success rate (96% group 1 vs 86% group 2, p = 0.17), long-term success rate (91% group 1 vs 89% group 2, p = 0.76), or major complications (8.0% group 1 vs 2.3% group 2, p = 0.39) were found. There were no procedure-related deaths in either group. These data suggest that, in two large volume electrophysiology centers, the procedural risks and outcomes of RFA are similar between patients weighing less than 15 kg and those between 15.1 and 20 kg.