A 2-year-old male underwent Fontan procedure as the third stage palliative repair for hypolastic left heart syndrome. On postoperative day number 15, he developed atrial flutter unresponsive to multiple attempts at cardioversion and therapeutic levels of digoxin and procainamide. Oral propafenone was administered and within 6 hours no further arrhythmias or side effects were noted. This case suggests that oral propafenone can be used with efficacy for the treatment of atrial flutter encountered in the immediate postoperative period following the Fontan procedure.