Participating Members of the Pediatric Catheter Ablation Registry* Background-The objective of this study was to determine the indications, the safety, and the efficacy of pediatric radiofrequency catheter ablation (RFCA) in infants. Methods and Results-Data from the pediatric RFCA registry were reviewed. Between August 1989 and January 1999, 137 infants, defined by age 0 to 1.5 years (median 0.7 years; weight 1.9 to 14.8 kg, median 10 kg), underwent 152 procedures in 27 of 49 registry centers (55%), compared with 5960 noninfants undergoing 6610 procedures during a comparable period. Structural heart disease was present in 36% of infants, compared with 11.2% of noninfants (PϽ0.0001). RFCA in infants was performed more commonly for drug resistance or life-threatening arrhythmias than in noninfants. No differences were found between infants and noninfants in success for all tachycardia substrates (87.6% versus 90.6%, Pϭ0.11), for single accessory pathways (94.5% versus 91.5%, Pϭ0.4), or for total (7.8% versus 7.4%, Pϭ1) and major (4.6% versus 2.9%, Pϭ0.17) complications. Neither success for infants with a single accessory pathway nor complications for the entire infant group were related to weight, age, center size, or the presence of structural heart disease. Centers that performed infant procedures, however, enrolled more patients overall in the registry than those that did not perform infant procedures, and successful procedures in infants were performed by more experienced physicians than failed procedures. Conclusions-Compared with noninfants, RFCA in infants is usually performed for drug resistance or life-threatening arrhythmias, often in the presence of structural heart disease. The data support the use of RFCA by experienced physicians in selected infants. (Circulation. 2001;104:2803-2808.)