OBJECTIVE: Fetal tachycardia is a condition associated with congestive heart failure and development of fetal hydrops, which may result in neurological morbidity and mortality. The aim of this study was to investigate the long-term outcome of hydropic fetuses. METHODS: This was a retrospective study on cognitive and neurological functioning of 11 infants, aged 6 months to 12 years, who experienced fetal tachycardia complicated by hydrops. RESULTS: Seven fetuses had supraventricular tachycardia (SVT), three had atrial flutter (AF) and one had ventricular tachycardia (VT). Nine fetuses converted to sinus rhythm within a mean time of 8.2 days of presentation; resolution of hydrops was achieved in six of these patients in a mean time of 8.8 days. Mean gestational age (GA) at birth was 35 + 4 weeks. Neonatal cranial ultrasound was normal in seven infants and all but one of these were normal at follow-up: one infant who initially had no abnormalities developed multiple cerebral lesions as a result of a malignant long QT syndrome (LQTS) and died at the age of 2 years. Three infants had periventricular echogenicity (PVE) on neonatal cranial ultrasound, associated with a pseudocyst in one infant. The remaining infant showed a parenchymal hemorrhage of antenatal onset, seen as a porencephalic cyst at birth. One of these infants was normal at follow-up, one died 2 days after birth and two infants had neurological abnormalities at follow-up, consisting of mild hemiplegia with normal cognitive function in one, and a cognitive developmental delay in the other. CONCLUSIONS: In this study, neurological outcome was good in eight out of 11 infants. Initiation of therapy should not be withheld or delayed on the assumption of poor neurological outcome.