We describe our experience with prenatal diagnosis of sex differentiation disorders, with focus on the role of ultrasound scans for coherent assessment of prenatal diagnosis. Over a 5-yr period all cases suspected of sexual ambiguity based on abnormal ultrasonographic scans (US) or US/genotype US discrepancy were evaluated prenatally by three modalities: 1) repeated fetal US; 2) genetic studies, primarily karyotype and fluorescence in situ hybridization analysis of sex-determining region on the Y gene (SRY); and 3) hormonal assays of amniotic fluid. Of approximately 10,000 gestations, 16 fetuses underwent prenatal evaluation. Twelve were referred because of an abnormal US and 4 because of genotype-phenotype discrepancy. Five fetuses were diagnosed with female pseudohermaphroditism (21-hydroxylase deficiency in 3 and urorectal septum malformation sequence in 2). Four fetuses were diagnosed with male pseudohermaphroditism (1 with steroid sulfatase deficiency, 1 with presumed camptomelic dysplasia, and 2 undetermined). Five cases had chromosomal abnormalities, and 2 had 46,XX+SRY sex reversal. In all genetic females the uterus was observed on US. In 11 cases initial US scan was performed at 13-15 wk; in 7 of 11, although the initial scan was normal, a repeated scan later in gestation revealed an abnormality. Repeated US scans performed at 13-15 and 22-24 wk gestation are a helpful tool in prenatal diagnosis of sex differentiation disorders. Our data suggest that both size and structure anomalies of the reproductive structures may evolve throughout pregnancy, and that they represent a developmental biological process rather than a single nonprogressive pathological event. US scan after approximately 19 wk enables detection of the uterus and provides pivotal information in cases of ambiguity. If the uterus appears normal, the most likely diagnosis is a virilized karyotypic female. Prenatal diagnosis allows for early parental counseling and anticipation of medical management postnatally.