It has been established that the finding of a fetal choroid plexus cyst confers an increased risk of aneuploidy, predominantly trisomy 18. In view of this association many investigators have advocated amniocentesis when a choroid plexus cyst is identified by prenatal ultrasonography. A risk/benefit and economic analysis of such a policy is presented. When the following factors are taken into account-published data on the frequency of isolated choroid plexus cysts in second trimester fetuses, the risk of trisomy 18 in these fetuses, the population frequency of trisomy 18, the natural history and prognosis for these infants, and the risks and cost of amniocentesis--it is concluded that performing amniocentesis on second trimester fetuses with choroid plexus cysts is neither cost effective nor beneficial. For the detection of one infant with trisomy 18 that will survive past five months, 25 normal fetuses would be lost owing to amniocentesis. Identification of trisomy 18 by determining the presence of choroid plexus cysts in fetuses is not a worthwhile screening policy.