Real-time directed M-mode echocardiography permits analysis of atrial and ventricular mechanical systole and allows inference of the type of arrhythmia present. Accurate diagnosis in cases of fetal tachyarrhythmia is of vital importance when therapy is considered, and in planning further management and delivery of an affected infant. Fetal tachyarrhythmia may be life-threatening especially when fetal hydrops is present and aggressive therapy is mandatory in these cases with digoxin being the drug of choice. In our series the incidence (8.3%) of congenital heart disease was as expected, but the incidence (54%) of atrial flutter was surprisingly high. The prognosis was dependent on the presence or absence of fetal hydrops and was not influenced by the type of arrhythmia or gestational age.