Between January 1, 1993, and April 30, 1996, authors treated 23 fetuses with severe rhythm disturbances in their Department. The correct diagnosis was made by fetal echocardiography. They had 15 tachyarrhythmic and 8 bradyarrhythmic patients. They found hydrops fetus at 7 patients because of atrial flutter (2 fetuses), supraventricular tachycardia (4 fetuses) and severe bradycardia (1 fetus). They treated successfully 13 patients with antiarrhythmic therapy given to the mother. They had 1 intrauterine death (treated because of bradycardia) and 1 neonatal death (hydropic because of supraventricular tachycardia). The causes of severe bradycardia were maternal antibody (3 fetuses), cardiac malformation (3 fetuses) and large number of blocked atrial extrasystoles. The prognosis of fetal tachycardia is good even in cases of fetal hydrops. The prognosis of bradycardia due to heart abnormalities is poor.