There are limited reports in the literature that address the unique issues related to cardiac catheterization on extracorporeal membrane oxygenation. Since 1990, cardiac catheterization has been performed on nine patients on extracorporeal membrane oxygenation. The median age was 10 months (range four days to 18 years) and the median weight was 4 kg (range 2.3–73.8 kg). The indications for catheterization were to relieve pulmonary edema in five patients with severe left ventricular dysfunction, and in one instance each, evaluation of probable anomalous left coronary artery, tetralogy of Fallot associated with a diaphragmatic hernia, possible pulmonary venous abnormalities, and postoperative evaluation after an arterial switch. Venous access was obtained through a preexisting femoral venous line in six patients, and through a cutdown of the saphenous or femoral vein in the other three. Interventional catheterization procedures were performed in six of the nine patients in this group. These consisted of blade atrial septostomy followed by balloon septostomy or dilation of the atrial septum in four patients, balloon septostomy alone in one, and placement of a catheter from the left atrium to the circuit used for extracorporeal oxygenation in another. A transseptal puncture was performed in five of them. Adequate decompression of the left atrium with relief of pulmonary edema was achieved in five patients. Six of the nine children in this group died, although there were no complications or deaths related to the catheterization. Children in cardiopulmonary failure on extracorporeal support represent a population at high risk. Diagnostic cardiac catheterization and atrial septostomy to decompress the left heart, when indicated, can be performed safely in such patients.