Zerres et al. (1992) state on the basis of six cases reported in the literature with trisomy 21 and choroid plexus cysts that "prenatally diagnosed choroid plexus cysts should in all cases be regarded as an indication for prenatal chromosomal diagnosis because additional abnormalities in trisomy 21 are sometimes absent in the second trimester" (emphasis added). But the fact that some reported trisomy 21 cases have choroid plexus cysts but no other ultrasonically detectable abnormalities is of uncertain relevance to clinical practice. One needs pertinent data on controls. The authors' reported data indicate only that the presence of choroid plexus cyst is at best an equivocal finding. They report that of 823 fetuses exhibiting developmental abnormalities defined as "growth retardation/malformation," 168 (20.4%) had a cytogenetic disorder. In the 25 of the 823 in which choroid plexus cysts were diagnosed, they report 5 (20%) in which there was a cytogenetic disorder. Thus, one may calculate that in those without choroid plexus cyst but with growth retardation/malformation, the proportion with cytogenetic abnormally actually is 163/798 = 20.4%, or actually slightly higher than in those with such cysts. This result casts significant doubt on the clinical utility of observation of the cyst, or the authors' conclusion cited above. Their data do tend, however, to support the inference that the presence of "abnormalities" in addition to choroid plexus cyst in those with "growth retardation," is suggestive of a cytogenetic defect, but primarily trisomy 18. Of the 11 with such additional abnormalities 5 (45%) had a cytogenetic defect, (4 with 47 + 18, one with 47 + 21), whereas one may calculate from the authors' data that none (0/14) of those without other defects had normal karyotype. This difference has a two-tailed probability of about 0.002, and is thus nominally significant. Unfortunately, interpretation of even these data from a clinical perspective requires knowledge of the level of intensity with which they scanned the ultrasound after finding choroid plexus cysts, and when in gestation malformation was noted. The likelihood of recognition of fetal malformation in ultrasound varies markedly with the size of the lesion, knowledge of the region likely to be affected, and the age of the fetus. Some abnormalities may be very difficult to diagnose and only recognized retrospectively on ultrasound after knowledge of presence of the malformation, or only at late stages of pregnancy, when, in many jurisdictions, a patient may no longer have the option to terminate pregnancy. At best the results suggest that in a conceptus with choroid plexus cyst and growth retardation, the presence of another malformation on ultrasound may increase the likelihood that a karyotypic abnormality is present. The precise nature of this increase in likelihood cannot be evaluated from the data.