A woman presented during two pregnancies (at 25 and 23 weeks' gestation, respectively) because the fetuses had rapid, irregular tachycardia and hydrops. After maternal drug treatment and achievement of slower fetal heart rates, the hydrops gradually resolved. Both babies were born full term with continuing atrial fibrillation. In the first, an ectopic atrial rhythm was temporarily achieved during high dose flecainide treatment but, in the younger sibling, all medications and repeated cardioversions failed even temporarily to convert the atrial fibrillation with an almost isoelectric baseline in ECG to sinus rhythm. Good rate control has been achieved with digoxin in both patients. No infective, immunological, or structural cause was found in either case, and thus an inherited aetiology is probable.