Persistent fetal tachycardias are known to have an adverse effect on fetal outcome. The outcomes of intermittent fetal tachyarrhythmias over a 12-year period at a tertiary fetal cardiology center were studied. Main outcome criteria included control of arrhythmia and death during the prenatal or postnatal period. A total of 28 fetuses had an intermittent tachyarrhythmia: 4 had intermittent atrial flutter and 24 had supraventricular tachycardia. At the time of presentation 14 fetuses were hydropic, and in 5 of the 14 an arrhythmia had not been noted prior to referral. Of the 28 fetuses, 23 were treated by drug administration to the mother. Control of arrhythmia was achieved in 10 of 11 (91%) nonhydropic fetuses and 8 of 12 (67%) hydropic fetuses, with resolution of hydrops in four cases. In the overall group there was one intrauterine death, two neonatal deaths, and one infant death, all of which occurred in the hydropic group. The arrhythmia recurred postnatally in 11 of 23 (48%) fetuses. We conclude that intermittent tachyarrhythmias may have a deleterious effect on the fetus with a significant risk of death pre- or postnatally. The fetus with nonimmune hydrops should be evaluated for a cardiac cause. Maternal antiarrhythmic therapy is indicated for intermittent fetal tachyarrhythmias. There is a high risk of recurrence of the arrhythmia during infancy, particularly if hydrops was documented during the prenatal period or if Wolff-Parkinson-White syndrome is diagnosed. Fetal echocardiography is a useful tool for diagnosis and for monitoring the progress of the fetus.