We report the unexpected occurrence of a lethal arrythmia in an 11 month-old boy who presented varicella. The disorder was found to be a junctional ectopic tachycardia due to a zosterian myocarditis confirmed at autopsy. This exceptional association of varicella, myocarditis, and junctional ectopic tachycardia gives us the opportunity to discuss on the diagnosis and treatment of this uncommon and serious arrythmia (regular tachycardia with normal QRS morphology, ranging 200-250 per min, with atrioventricular dissociation or retrograde conduction to the atria).