Atrial flutter and atrial fibrillation present difficult problems in management, particularly during infancy. The clinical experience with 6 personally observed and 30 reported cases of infantile atrial flutter is described. Two types of flutter are distinguished: type I (congenital), which occurs prior to birth or within the first week of life, responds to digitalis in about half the cases, and has a nearly equal sex distribution; and type II (paroxysmal), which occurs predominantly in males and infrequently responds to digitalization. The prognosis is poor if either atrial fibrillation or a coexistent congenital cardiac defect is present. Even in patients without these conditions, the prognosis is guarded. Digitalis is the treatment of choice since it induces a sinus rhythm in many cases and slows the ventricular response in the remainder. Cardioversion, although tried in only one patient, should be a useful therapeutic tool.