From January 1986 to August 1990, a fetal arrhythmia was diagnosed in 97 pregnant women referred to our unit. Atrial or ventricular extrasystoles were the most frequent rhythm disturbance encountered (71%). They were always well tolerated and all disappeared during the perinatal period. Tachycardia was found in 16 fetuses; 6 had a supraventricular reentrant tachycardia, 6 an atrial chaotic tachycardia, 1 an ectopic atrial tachycardia and the remaining 3 an atrial flutter. A congenital heart malformation was present in 4 fetuses. The arrhythmia induced hydrops fetalis in 25% of cases. One hydropic fetus died in utero and another needed premature delivery. Bradycardia was diagnosed in 12 cases, 3 had benign atrial blocked extrasystoles, 3 others sinus bradycardia due to fetal distress and 6 atrio-ventricular block. Atrio-ventricular block was associated with congenital malformation in 4 cases (66%). All these fetuses were hydropic and died. The 2 fetuses without cardiac malformation tolerated well their bradycardia during fetal life. Fetal arrhythmia is not rare, but in most cases is benign. Sustained tachycardia requires prompt treatment, because when hydrops fetalis appears the prognosis is worse. The major prognostic factor for atrio-ventricular block is the association with a cardiac malformation.