The antenatal diagnosis of fetal arrhythmias can be a difficult proposition. This paper reports 234 cases of fetal cardiac arrhythmias. One hundred and ninety-four of these were isolated extrasystoles which resolved spontaneously in all except 2 cases. There were 9 patients with complete heart block. Three of these patients had structural anomalies and hydrops. Four other patients with heart block had evidence of maternal collagen disease. Sustained supraventricular tachyarrhythmias are the most common clinically important arrhythmias. Most of these can be successfully controlled with digoxin alone or with verapamil or propranolol. Atrial flutter is more difficult to control than supraventricular tachycardia.