Cardiac catheterization has been utilized rarely in children on extracorporeal membrane oxygenation (ECMO). We performed a retrospective review of 15 children with congenital heart disease who had undergone catheterization while on ECMO from December 1990-December 1995. The procedures, including four interventions, were successful in all patients with adequate evaluation of clinical questions. Unexpected diagnostic information of clinical importance was obtained in 40%, and clinical management of patients was significantly altered in 73%. All patients tolerated the procedure and transport well. The only significant complication was a retroperitoneal hemorrhage in one patient after approximately 12 hr. Although no patients died at catheterization, overall survival was poor, with 50% weaning from ECMO, 29% surviving to discharge, and 14% surviving at follow-up. We conclude that diagnostic and interventional catheterization may be performed in patients on ECMO with acceptable morbidity and mortality; however, long-term survival in this population is poor.