Choroid plexus (CP) cysts have been associated with trisomy 18, although most fetuses with CP cysts are normal. Since many fetuses with trisomy 18 have other sonographic abnormalities, the necessity of obtaining a karyotype for all fetuses with isolated CP cysts remains controversial. The authors prospectively studied 234 second-trimester fetuses with sonographically discovered CP cysts. Two hundred twenty of them had no other sonographic findings. None of these 220 normal fetuses had evidence of aneuploidy at amniocentesis or an anomaly at birth. Fourteen fetuses had major anomalies detected in utero: 11 had trisomy 18, one had triploidy, and two had normal karyotypes but were structurally abnormal. While size and bilaterality of the CP cysts were not helpful in predicting aneuploidy, the meticulous anatomic survey of fetuses with CP cysts allowed successful identification of all aneuploid fetuses. These data show that the yield of abnormal karyotypes in fetuses with isolated CP cysts is low and may not justify the risk of amniocentesis.