INTRODUCTION: Fetal tachycardia may lead to an increased pre- and postnatal morbidity and mortality rate particularly if it is complicated by cardial decompensation and hydrops fetalis. AIM AND METHODS: In this study 33 fetal tachycardia cases diagnosed and treated between 1993 and 2004 in the fetal echocardiography unit of the I. Department of Obstetrics and Gynecology of the Semmelweis University, Budapest are reviewed. The data of postnatal care of the newborns delivered in the author's department from these pregnancies, and the follow up data provided by the National Institute of Cardiology are examined as well. RESULTS: Mean gestational age at diagnosis of fetal tachycardia was 30 weeks (21-41 weeks). The tachyarrhythmias were classified into atrial flutter (n = 8), supraventricular tachycardia (n = 18), arrhythmia absoluta (n = 5), parasystole (n = 1) and brady-tachyarrhythmia (n = 1). Six cases were complicated by hydrops fetalis, 13 cases by cardial dysfunction. Transplacental antiarrhythmic therapy was applied in 22 cases, in 8 cases the newborns were delivered because of advanced gestational age, in 3 cases tachyarrhythmia resolved spontaneously or therapy was not indicated. The drug of first choice for transplacental therapy was digoxin, which was combined with amiodarone or verapamil (n = 10). Transplacental therapy led to cardioversion in 13/22 cases. The outcome of the 33 examined pregnancies was live birth in 27 cases, in utero death in 3 cases and 3 newborns were delivered elsewhere. The postnatal documentation of 24 newborns out of the 27 born in the author's department is available. At the time of birth 15/24 newborns were in sinus rhythm--out of whom 5 developed tachyarrhythmia later during the neonatal period--, 9/24 were tachycardic. Out of the 14 cases of tachyarrhythmia detected in the neonatal period altogether 3 resolved spontaneously, in 7 cases antiarrhythmic therapy was successful, in 4 cases unsuccessful. In 2 of these latter cases electrical cardioversion led to sinus rhythm. Neurological disorder was not detected in any case. In the early postnatal period 2 in utero severely decompensated newborns died. The follow-up data of 10 children is available, the follow-up period ranges between 6 weeks and 5 and a half years. All 5 children with history of supraventricular tachycardia are in sinus rhythm, 3 of them after suspending antiarrhythmic treatment, while the other 2 still on antiarrhythmic medication. Four out of 5 children with history of atrial flutter are in sinus rhythm, 2 of them left antiarrhythmic therapy, and 2 of them still take antiarrhythmic agents after electrical cardioversion. The atrial flutter of a 3 month old child could not be controlled yet permanently, despite several drug combinations applied. CONCLUSIONS: Survival and late prognosis of tachycardic fetuses treated in utero is good. A prospective study of even more cases is required to establish uniform therapeutic guidelines and to provide appropriate follow-up data.