Our aim was to evaluate the prevalence of trisomy 18 in the setting of isolated fetal choroid plexus cysts and then to consider the risk of trisomy 18 versus the risks of genetic amniocentesis. Fetuses with choroid plexus cysts were prospectively obtained from a total mid-trimester population of 18861 fetuses with known outcomes. If the fetuses had trisomy 18, they were part of the study group and part of the control group if they had normal karyotypes. Scans were retrospectively reviewed for the characterization of cysts according to size, laterality, and appearance (simple or complex echo patterns). Chi-square analysis of contingency tables of results was performed. 208/18861 (1.1 per cent) fetuses had choroid plexus cysts. 201/208 (96.6 per cent) were normal fetuses or newborns, while 7/208 (3.4 per cent) of the fetuses with choroid plexus cysts had trisomy 18. Overall, 16 fetuses had trisomy 18 and seven (44 per cent) of these had choroid plexus cysts. 0/16 fetuses had choroid plexus cysts as the only sonographic finding. Although laterality or complexity of the cysts did not correlate with the presence or absence of a cytogenetic abnormality, cysts > or = 10 mm were more often associated with trisomy 18 than with a normal karyotype (P < 0.01). We conclude that the discovery of choroid plexus cysts in otherwise normal fetuses in the late second trimester does not by itself justify the risks of genetic amniocentesis.