Objective. Our objective was to investigate the outcome of neonates with idiopathic polyhydramnios in the first year after birth. Methods. All patients diagnosed in the Erasmus Medical Centre Rotterdam and the University Medical Centre Utrecht between January 2000 and April 2005 with idiopathic polyhydramnios were studied. The outcome variables included mode of delivery, pre-term delivery, birth weight, macrosomia, large-for-gestational-age (weightp90), neonatal or infant mortality and infant morbidity at 1 year after delivery. These were related to antenatal findings, including the onset of polyhydramnios and ultrasonographic evidence of macrosomia (estimated fetal weightp90). Results. Polyhydramnios was diagnosed at a mean gestational age (s.d.) of 31.04.9 weeks. The mean gestational age at birth (s.d.) was 37.93.7 weeks. Macrosomia at birth was present in 25.3% (22/88). Sixty-three of 88 infants were in good health 1 year after birth; 20 infants had an abnormality and 5 children had died. Macrosomia and detection of polyhydramnios in the third trimester was associated significantly with a good outcome. Conclusion. In neonates with idiopathic polyhydramnios, abnormalities were detected during the first year of life in 28.4%. Detection of polyhydramnios in the second trimester and low or normal birth weight are risk factors for associated abnormalities.