We describe a case of a patient with cisplatin-induced hypomagnesemia who suffered brief asystole during an episode of gastroenteritis. Structural heart disease was excluded. The patient achieved complete clinical recovery after short-term administration of intravenous magnesium supplementation. Cisplatin should be considered a cause of hypomagnesemic-related cardiac dysrhythmia. Magnesium deficit may increase myocardial electrical instability and thus, the risk of life-threatening arrhythmias and sudden death. Long-term serum electrolyte measurement and appropriate replacement of magnesium are recommended. Pediatr Blood Cancer 2007;49:867–869. © 2006 Wiley-Liss, Inc.