A newborn infant presented with a life-threatening cardiac arrhythmia. Echocardiography showed a large cardiac mass growing exophytically in the region of the left atrial appendage extending along the left ventricular wall but showed no clear tissue plane between the mass and the left ventricular wall. Based on the echocardiogram, surgical resection might damage the left ventricular wall. Magnetic resonance imaging demonstrated a clear plane of demarcation, making surgical resection a viable life-saving option.