Thirty fetuses with cardiac arrhythmias were referred for ultrasonography. This included cross sectional and M mode echocardiography and pulsed Doppler analysis of the fetal heart. Three types of arrhythmias were observed: ectopic beats, tachyarrhythmias, and bradycardia. Ectopic beats were associated with cardiac structural abnormalities in two cases, resulting in fetal death in one. Tachycardia was not associated with structural defect, but death from cardiac failure occurred in one patient. Transplacental treatment for tachyarrhythmia was not successful in our experience. In the group with bradycardia four cases had congenital cardiac abnormalities and the mortality rate was 50%. When a fetal cardiac arrhythmia has been established careful structural and rhythm analysis is of vital importance in facilitating prognosis, planning of time and mode of delivery, and monitoring of transplacental treatment where indicated.