The ketogenic diet is a high-fat, low-carbohydrate, adequate-protein diet that is used to decrease the frequency of seizures in patients who have refractory epilepsy. Despite its positive effects in some patients, there are potential adverse effects. Two complications related to the ketogenic diet are selenium deficiency, which has been associated with impaired myocardial function, and QT prolongation as documented on electrocardiography. Reported here are two cases of death in a child on the ketogenic diet for seizure control. In case 1, the child who died of complications related to torsade de pointes, with documented QT prolongation; post mortem examination revealed selenium-deficiency cardiomyopathy. In case 2, a child experienced QT prolongation while on the ketogenic diet and later died suddenly at home. Both children exhibited selenium deficiency. These two cases suggest that patients on the ketogenic diet require monitoring of the QT interval by electrocardiography, myocardial function by echocardiography, and selenium levels before and during the ketogenic diet.