The results of the ultrasonographic determination of fetal gender in utero in 722 fetuses (13-35 weeks' gestation) are described, demonstrating that fetal genitalia can be seen in 60.5 per cent of those examined before the eighteenth week, and in 100 per cent of those examined twice or once after 20 weeks of gestation. All errors (3.04 per cent) of gender assignment occurred before the twenty-fourth week. When the fetus was examined for the first time at 17 weeks, the genitalia were visualized and correctly diagnosed in 282 males and 155 females; nine males and 13 females were incorrectly diagnosed. Ultrasonographic determination of fetal gender in utero is an integral part of the prenatal diagnosis of sex maldefinition, testicular feminization, and campomelic dysplasia. It has proved to be a reliable marker in determining whether each sac has been sampled in multiple pregnancies (when each fetus is in a different sac) if ultrasonographically assigned sex per twin corresponds to its karyotype. The determination of fetal gender in utero by ultrasonography allows for gender selection; some of its ethical implications are considered.