Current literature suggests that amniocentesis be performed on fetuses with simple choroid plexus cysts only when such cysts are 1.0 cm or greater in diameter and bilateral. At retrospective analysis of 3,769 patients, choroid plexus cysts were noted in 87 (2.3%), representing a rate three times greater than that of previous reports. Eight-three patients underwent amniocentesis. Six (7.2%) had abnormal karyotypes. Four patients had the commonly associated chromosomal abnormality trisomy 18. Two had karyotypes not usually associated with this problem: mosaic Turner syndrome and trisomy 21. Of the six patients with abnormal karyotypes, one had a 4-mm-diameter unilateral choroid plexus cyst and three had bilateral cysts of 3-5 mm. Only one patient with a 16-mm cyst had any associated structural abnormality discovered at rigorous ultrasound examination. Karyotyping may be necessary in fetuses with small choroid plexus cysts. Deciding which patients should be encouraged to undergo amniocentesis is made more complex by these data.