We determined the type and frequency of abnormal sonographic findings in 187 Down syndrome fetuses. Examinations were performed transvaginally or transabdominally between 9 and 28 weeks' gestation. Consecutive scans performed prior to knowledge of the fetal karyotype (n=144) were analysed separately for one of the participating centres. In 93 fetuses (49β’7 per cent), a total of 138 abnormalities were observed. The most commonly detected anomalies were cystic hygroma and increased nuchal fold thickness (30β’5 per cent), hydrops (9β’6 per cent), cardiac defects (7β’5 per cent), pyelectasis or hydronephrosis (5β’9 per cent), echogenic bowel (4β’8 per cent), and a large variety of internal organ abnormalities (16β’0 per cent) which are not typically associated with Down syndrome. Two anomalies or three anomalies in the same fetus were observed in 21 and 5 fetuses, respectively. No patterns of concurrent malformations were apparent among these fetuses. Sensitivity for Down syndrome detection by ultrasound scans performed without knowledge of the fetal karyotype was 24β’1 and 42β’6 per cent before 13 weeks and between 14 and 23 weeks, respectively. We conclude that structural abnormalities are frequently observed in Down syndrome fetuses, but many sonographic findings are not typically associated with this syndrome.