The biometric and structural sonographic features of 95 second-trimester fetuses with Down's syndrome were evaluated to determine whether affected male fetuses differed from affected females. There were 54 male and 41 female fetuses with Down's syndrome studied. A shortened femur was identified in 28/54 (52%) males compared with 19/41 (46%) affected females (NS). A thickened nuchal fold was identified in 19/54 (35%) of males vs. 20/41 (49%) of females. Renal pyelectasis was seen in 7/54 (13%) males and 8/41 (19%) females. A heart defect was seen in 8/54 (15%) males and 7/41 (17%) females. Ventriculomegaly was identified in 6/54 (11%) males and 3/41 (7%) females with Down's syndrome. There were no statistically significant differences in the incidence of the sonographic findings when male and female Down's fetuses were compared. Our data show that the criteria for evaluation of sonographic markers for the identification of second-trimester fetuses with Down's syndrome should be the same in male and female fetuses.