OBJECTIVE To examine the possible association between increased fetal nuchal translucency thickness at 10-14 weeks and congenital diaphragmatic hernia. METHODS This was a multicenter ultrasound screening study for chromosomal defects in singleton pregnancies by a combination of maternal age and fetal nuchal translucency at 10-14 weeks' gestation. The prevalence of diaphragmatic hernia diagnosed prenatally or postnatally was calculated in the chromosomally normal group and in those pregnancies resulting in live births with no dysmorphic features suggestive of a chromosomal abnormality. We calculated the sensitivity of nuchal translucency above the 95th centile of the normal range in the detection of diaphragmatic hernia and the possible prognostic value of increased nuchal translucency in the prediction of outcome. RESULTS There were 78,639 pregnancies presumed to be normal chromosomally, including 19 with diaphragmatic hernia. In four cases, the parents opted for termination of the pregnancy. The other 15 pregnancies resulted in live births; nine infants survived after successful surgical repair of the hernia, but six neonates died because of pulmonary hypoplasia. At the 10- to 14-week scan, the fetal nuchal translucency was above the 95th centile for crown-rump length in seven (37%) cases of diaphragmatic hernia. The translucency was increased in five of the six cases that resulted in neonatal death, compared with two of the nine survivors (Z = 2.32, P < .05). CONCLUSIONS The prevalence of diaphragmatic hernia in chromosomally normal fetuses is about one in 4000, and nearly 40% of affected fetuses have increased nuchal translucency at 10-14 weeks' gestation. Increased nuchal translucency may be a marker of intrathoracic compression-related pulmonary hypoplasia.