Serelaxin has been studied in trials in adults with acute heart failure, but not in children. We report the first compassionate use of serelaxin in an infant. A 6-month-old girl with dilated cardiomyopathy was placed on extracorporeal membrane oxygenation following cardiac arrest unresponsive to medical treatment. Extracorporeal membrane oxygenation weaning failed despite maximal ino-dilator therapy. During the 48-hour infusion of serelaxin, we observed marked improvement in brain natriuretic peptide, left ventricular systolic function, and dilatation. The patient was successfully weaned from extracorporeal membrane oxygenation 24 hours later. The child died after a second extracorporeal membrane oxygenation run owing to sepsis.