We sought to evaluate the accuracy of the genetic sonogram scoring index in detecting Down syndrome fetuses in a high-risk population. Women referred for genetic sonogram scoring index based on increased risk for aneuploidy were prospectively evaluated. Each fetus was assigned a score based on our previously published genetic sonogram scoring index. A score of 1 or greater was an indication for karyotype determination. Of 1076 patients (1118 fetuses) in the study group, follow-up data were available for 1030 (92%), eight of whom had Down syndrome (prevalence 0.8%). Three fetuses with Down syndrome had a genetic sonogram scoring index of 0, and for one of these the mother was 42 years old. One affected fetus had a score of 1 and the other four had scores of 2 or greater. Using a genetic sonogram scoring index of 1 or greater (age not considered), five of eight fetuses with Down syndrome (62.5%) were identified, as well as 150 of 1030 (14.6%) of normals. Down syndrome was undetected in three fetuses of 1030 (1 in 343) or 0.29%. In addition, when all women 40 years old or older underwent karyotype testing regardless of their sonographic findings, six of eight Down syndrome fetuses were identified (75%), as well as 271 of 1030 (26.3%) of normals. The number of cases of Down syndrome not detected was two in 1030 (1 in 515) or 0.19%. In conclusion, the genetic sonogram scoring index was used to identify approximately 75% of fetuses with Down syndrome, with amniocentesis being recommended in 26.7% of a high-risk population.