Cardiac channelopathies caused by SCN5A mutation are well tolerated by most patients. However, the dramatic presentation of a previously healthy 4-month-old girl with life-threatening arrhythmias and the subsequent findings in the child and her family provide evidence that loss-of-function sodium channel mutations can present very early in life. An SCN5A mutation was detected in the infant, her brother, and their father. Both the siblings manifested recurrent serious arrhythmias during febrile episodes, which followed immunization, as well as fever of nonspecific origin. Management consisted of prompt antipyretic measures, hospitalization with vigorous monitoring during immunization and febrile episodes, and prevention of tachycardia-induced conduction disturbance with β-blockers.