Choroid plexus cysts are more common in fetuses with chromosomal aneuploidies, particularly trisomy 18. Although it is accepted that the risk of karyotypic abnormality justifies amniocentesis when associated abnormalities are present, disagreement continues as to the risk of trisomy 18 in a fetus with an isolated choroid plexus cyst. We propose consideration of maternal age and multiple-marker screening for chromosomal aneuploidy in the assessment of risk. Bayesian statistical modeling was used to calculate the risk of trisomy 18 from age-related risk figures for trisomy 18 and the incidence of isolated choroid plexus cysts in fetuses with trisomy 18. The risk was further modified on the basis of the ability of multiple-marker screening to detect fetuses with trisomy 18. From risk estimates calculated across maternal ages 20 to 45 years, the risk of trisomy 18 does not approach that of amniocentesis until a maternal age of > or = 37 years. Therefore in the presence of an isolated choroid plexus cyst and normal multiple-marker screen results amniocentesis is justified only in the patient with advanced maternal age.