Cystic tumour of the atrioventricular node (CTAVN) is a rare type of cardiac tumour. We report a case of a previously healthy nine year-old girl, presenting with syncope caused by complete AV block. Despite intensive treatment, she died of multiple organ failure after two days. Infectious and congenital AV block were ruled out. Histology of the conduction system showed that a microscopic CTAVN was the cause of death. CTAVN is a rare cause of AV block and cardiac arrest, but should be considered in cases of unexplained death.