In newborns, atrial flutter is a rare, but severe condition. Late diagnosis was usual and based on fetal ECG. Now, fetal ultrasonography can differentiate in utero atrial flutter and other supra ventricular tachycardias; so it can be treated before development of cardiac failure. Among numerous and various available drugs, digoxin is universally recommended, but the delayed effect and the maternal serum levels to obtain effective fetal concentration of digoxin are not well known. Furthermore, endogenous "digoxin-like" substances have been found in maternal blood in late pregnancy; this finding raises the problem of the validity of monitoring the treatment only by serum digoxin levels. A case of atrial flutter, diagnosed in utero by ultrasonography and treated by digoxin before development of cardiac failure is reported. Neonatal echocardiography showed an aneurysm of the atrial septum, which needed simple monitoring. Its responsibility in the pathogenesis of the arrhythmia is unclear. Numerous cases of aneurysm of the atrial septum have been reported in healthy adults without atrial arrhythmias.