The authors report a rare case of cannula thrombosis during extracorporeal membrane oxygenation (ECMO). A full-term newborn infant was successively placed on single-cannula veno-venous extracorporeal lung support and then on veno-arterial ECMO, because of persistent pulmonary hypertension. At 140 hours of ECMO, the infant displayed general cyanosis except in the right arm. Since asymmetric hypoxemia during ECMO may be related either to cannula malposition or to a tip thrombosis, a chest x-ray after contrast injection into the arterial line of the circuit was performed. It showed an opacification of the whole cannula but for the last distal centimeter, and of the vascular bed extending from the right subclavian artery. Cannula thrombosis was suspected and confirmed by removal of the arterial cannula. Demonstration of cannula thrombosis by opacification of the arterial line of the circuit indicates catheter removal.