Previous published reports have shown that fetuses affected with Down syndrome have normal biparietal diameter (BPD), high BPD-to-femur length ratio, and shorter femur length and occipitofrontal diameter. A retrospective comparison of the BPD, occipitofrontal diameter, cephalic index, femur length, and BPD/femur length ratio was made between 17 fetuses with Down syndrome diagnosed prenatally by second-trimester amniocentesis and 17 matched controls. The gestational age at sonographic evaluation in the two groups ranged from 15-23 weeks. The results of this study show no significant differences in the BPD, occipitofrontal diameter, cephalic index, femur length, and BPD/femur length ratio in the Down syndrome group when compared with controls. The sensitivity of BPD/femur length ratio as a screening technique for Down syndrome in this study was 18%, with a specificity of 94%. This sonographic method does not appear to be a useful screening parameter for Down syndrome.