To assess the value of second trimester genetic ultrasound for screening of Down syndrome conducted at Srinagarind Hospital, Khon Kaen, Thailand.The present study sample comprised of 4,033 pregnant women at high risk forfetal chromosomal abnormality, from 17th to 23th week, who had performed second trimester genetic ultrasound before genetic amniocentesis between September 1996 and December 2011. Archived medical records relating to results ofgenetic ultrasound and genetic amniocentesis were extracted and studied.Sensitivity ofgenetic ultrasound in the detection of fetal Down syndrome. Results: There were 3,966 chromosomally normal pregnancies (98.3%), 43 fetuses with Down syndrome (1.1%), and 24 fetuses with other chromosomal abnormality (0.6%). 30 of 43 (69.8%) fetuses with Down syndrome had abnormal genetic ultrasound. The overall sensitivity of second trimester genetic ultrasound for detecting Down syndrome was 69.8% with a false-positive rate of 50.4% and likelihood ratio of 1.38. Of all the sonographic markers, short femur, and short humerus indicated the highest sensitivity at 65.1% and 44.2%. According to likelihood ratio (LR+), chest abnormality, 2 vessel umbilical cord, andfacial abnormality including cleft lip and palate, have highest likelihood ratio (LR+) of 61.49, 46.12, and 46.10, and had sensitivity at 4.7%, 2.3%, and 2.3% respectively.The sensitivity of second trimester genetic ultrasound for detection offetal Down syndrome at Srinagarind Hospital was rather high, and probably is an alternative method ofprenatalprediction for high risk pregnant women who refused genetic amniocentesis.