Several ultrasonographic signs have been described in second-trimester Down's syndrome fetuses. Previously published reports have shown that fetuses affected with Down's syndrome have normal biparietal diameter (BPD), high BPD-to-femur length (FL) ratio, and low actual FL-to-expected FL ratio. A retrospective comparison of the BPD-to-FL ratio and actual FL-to-expected FL ratio was made between 3 fetuses with Down's syndrome diagnosed prenatally by second-trimester amniocentesis and 189 normal fetuses with gestational age varying from 13-25 weeks who were seen in the Ultrasound Unit of Department of Obstetrics & Gynaecology, Chulalongkorn Hospital between January 1, 1989 and May 31, 1990. The sensitivity of BPD-to-FL ratio and actual FL-to-expected FL ratio as a screening technique for Down's syndrome in this study was 66.7 and 100 per cent, with a specificity of 93.4 and 89.2 per cent respectively. These sonographic parameters appear to be a useful screening method for Down's syndrome.