Mendelsohn A, Dick M 2d, Serwer GA. C.S. Mott Children's Hospital, Ann Arbor, MI. J Pediatr 1991 Sep;119(3):386-91. To clarify the natural history of isolated (i.e., without associated congenital cardiac anomalies) atrial flutter in infancy, we reviewed the clinical course in nine patients who were seen with this arrhythmia in the first year of life (range 1 day to 4 months). Atrial flutter was identified by the typical sawtooth pattern in leads II, III, and aVF of the surface electrocardiogram or the pattern of atrial flutter on an atrial electrogram recorded through the esophagus. The mean cycle length of the atrial flutter was 151 msec (atrial rate 397 beats/min). Six of the nine patients had other perinatal problems, such as immune and nonimmune hydrops fetalis (two patients), pneumonia (one patient), anemia (five patients), or low birth weight (one patient). In all patients the rhythm reverted to normal, either spontaneously (two patients), with overdrive pacing (four patients), or after oral digoxin therapy (three patients). No consistent temporal relationship between digoxin administration and conversion was observed; conversion was instantaneous in the four patients who received atrial overdrive pacing. Four patients were discharged receiving digoxin therapy (6 months to 1 year). One patient had supraventricular tachycardia after discharge that was controlled with digoxin. No recurrence of atrial flutter was observed among the nine patients during a mean follow-up of 6.8 years (range 0.2 to 20 years). We conclude that isolated atrial flutter in infancy is rare, has a good prognosis, may be related to transient perinatal events, and often spontaneously converts to normal sinus rhythm; however, when it does not, it will respond to transesophageal pacing.