Fetal supraventricular tachycardia (SVT) can be successfully treated transplacentally, but in cases where fetal hydrops develops there is considerable morbidity and mortality. The present study was carried out to establish whether the introduction of flecainide altered obstetric management and fetal outcome. A retrospective analysis took place of 51 singleton pregnancies which were referred to the division of prenatal diagnosis because of fetal tachycardia between 1982 and 1993. SVT was documented in 50 out of 51 fetuses, one of which displayed a combination of extensive rhabdomyomas and severe hydrops and died shortly after referral. In the other fetus ventricular tachycardia was diagnosed. Of the remaining 49 fetuses, 14 did not receive any prenatal treatment, but nine needed postnatal treatment. Transplacental treatment of SVT took place in 35 fetuses, of which 22 presented without hydrops and 13 with hydrops. These subsets differed significantly with respect to restoration of normal sinus rhythm (73% vs. 30%; p < 0.001) and mortality (0% vs. 46%; p < 0.001). Digoxin was effective in restoring sinus rhythm in 55 per cent of the non-hydropic fetuses but in only eight per cent of the hydropic fetuses. Flecainide was effective in restoring sinus rhythm in all non-hydropic fetuses where digoxin treatment failed, and in 43 per cent of hydropic fetuses. Administration of flecainide resulted in a significantly reduced mortality (p < 0.001) compared with digoxin treatment. No adverse effects were seen. Postnatal anti-arrhythmic treatment was necessary in 23 infants. Treatment could be withdrawn within one year in all cases but one.