Those clinical and electrophysiologic features of paroxysmal atrial tachycardia (PAT) that appeared to be the cause of prenatal congestive heart failure (CHF) (hydrops fetalis) were evaluated in 12 neonates, ages 1 to 14 days, and compared with those occurring in 12 neonates, ages 2 to 34 days, who developed CHF from PAT diagnosed postnatally. Transesophageal electrophysiologic evaluation was performed after birth or at the time of PAT occurrence in the prenatal CHF group and at the time of CHF diagnosis in the postnatal CHF group. Before the electrophysiologic study, spontaneous PAT onset and termination were observed in all prenatal CHF neonates. In the postnatal CHF neonates, however, a single, prolonged episode of PAT was observed. During PAT, all neonates were observed to have a regular heart rate, normal QRS morphology and ventriculoatrial interval exceeding 80 ms. These findings suggested an orthodromic reciprocating tachycardia using an accessory atrioventricular connection as the tachycardia mechanism. In the 12 neonates with prenatal CHF, the PAT cycle length measured 243 ± 30 ms, whereas in the 12 neonates with postnatal CHF it measured 208 ± 19 ms (p < 0.003). Neonates with prenatal CHF secondary to PAT appear to develop CHF from multiple PATs recurring at relatively long cycle lengths (slow heart rates), whereas postnatal CHF neonates develop heart failure symptoms during a sustained tachycardia episode with relatively short cycle lengths (fast heart rates).