A prospective study was undertaken to evaluate the prevalence and significance of fetal choroid plexus cysts detected on screening ultrasound (US) scans. One hundred cases of fetal choroid plexus cysts were detected from 11,700 examinations: There were 95 healthy newborns, three newborns with trisomy 18, one newborn with syndactyly of the toes, and one intrauterine death. In the three neonates with trisomy 18 the cysts were large (greater than 1 cm in diameter), and additional abnormalities were detected. Although there is a clear association between fetal choroid plexus cysts and trisomy 18, amniocentesis or chorionic villus sampling should be reserved for high-risk patients with either large cysts or US evidence of other abnormalities.