To determine the prevalence, sonographic appearance, and clinical significance of fetal choroid plexus cysts, we analyzed the sonograms and clinical records of 17 fetuses with cysts. Fetal and maternal age, sonographic indication, cyst size and multiplicity, and evolution on serial studies were recorded. Fetal outcome was available in 16 cases by genetic amniocentesis (n = 5) or neonatal clinical records (n = 11). The prevalence of fetal choroid plexus cysts was 0.8% (17/2084) during a 40-month period. All cysts were initially identified on sonograms performed between 14 and 21 weeks. Cysts ranged from 3 to 11 mm in size and were bilateral in four (36%) of 11 cases in which both lateral ventricles were visualized. In nine of 10 cases with serial sonograms 2-21 weeks after the initial study, the cysts were no longer present. One fetus had a small cyst persisting at term. All five cases with genetic amniocentesis had normal chromosomes. The only phenotypic abnormality in the 11 cases with clinical follow-up was a small hemangioma of the chest wall. We conclude that most fetuses with isolated choroid plexus cysts have a normal outcome and that serial sonography for cyst evaluation is not useful in determining fetal prognosis.