Of 60 neonates who survived extracorporeal membrane oxygenation (ECMO) in our institution between June 1992 and March 1994, seven had either complete or partial superior vena cava (SVC) obstruction. When the patients with SVC obstruction were compared with those who had an echocardiogram after ECMO, no predisposing factors for the development of SVC thrombus could be found. Our data show that SVC thrombus may be a significant complication after ECMO.