Prenatal sonographic findings were reviewed in 47 consecutive fetuses with trisomy 18. One or more abnormalities, excluding choroid plexus cysts, were found in 39 fetuses (83%), including 21 of 29 (72%) fetuses examined between 14 and 24 weeks and all 18 (100%) fetuses examined after 24 weeks. Abnormalities most frequently detected before 24 weeks included cystic hygromas, nuchal thickening, and meningomyelocele. Intrauterine growth retardation, cardiac defects, and an enlarged cisterna magna were detected more frequently after 24 weeks than before 24 weeks (P < 0.05). Intrauterine growth retardation was the single most common abnormality, detected in 51% of all fetuses and 89% of fetuses examined after 24 weeks. Choroid plexus cysts were identified in 25% (12 of 47) of fetuses, including 11 of 29 (38%) fetuses seen before 24 weeks. Additional abnormalities were identified in 10 of 12 (83%) fetuses with choroid plexus cysts; one of the two remaining fetuses showed clenched hands and a cardiac anomaly at autopsy, and the other showed unusually large, multiple choroid plexus cysts. Awareness of the type of anomalies and the usual menstrual age of diagnosis in fetuses with trisomy 18 should improve patient counseling and prenatal detection of fetuses considered at risk for this disorder.