Background <p>Williams syndrome (WS) is a rare genetic multisystem disorder that often affects the cardiovascular system. However, an association with Ebstein’s anomaly (EA) is extremely uncommon. Previously, only two such cases have been reported. Here, we present a third patient with concurrent WS and EA, manifesting with life-threatening ventricular arrhythmias.</p> Case presentation <p>Our case involves an 11-year-old boy, diagnosed prenatally with EA and 3-months postnatally with WS on the basis of a 7q11.23 deletion. Over the course of follow-up since birth, life-threatening ventricular arrhythmias occurred, originating from a likely fibrosed Purkinje system possibly related to elastin haploinsufficiency. The resulting out-of-hospital cardiac arrest warranted implantation of an implantable cardioverter-defibrillator and antiarrhythmic medication.</p> Conclusions <p>Ventricular arrhythmia and out-of-hospital cardiac arrest constitute an unusual clinical presentation that has not previously been reported in cases with concurrent WS and EA, but may be attributed to elastin haploinsufficiency. This potentially fatal complication underscores the importance of vigilant clinical surveillance in patients with comparable congenital heart defects. Our case report adds further evidence to the limited literature describing congenital cardiac anomalies and associated complications in patients with WS.</p>