To determine the usual sonographic findings and clinical significance of the Dandy-Walker malformation (DWM) identified in utero, we reviewed seven proven cases diagnosed by prenatal ultrasound during a 6-year period. All patients demonstrated complete or partial absence of the cerebellar vermis and a posterior fossa cyst. Other central nervous system (CNS) findings included hydrocephalus (five cases) and agenesis of the corpus callosum (one case). Extra-CNS abnormalities were prospectively identified in 3 of 4 fetuses with multiple anomalies. Chromosomal analysis was performed on all four fetuses with multiple extra-CNS anomalies, yielding 2 abnormal karyotypes (trisomy 18 and mosaic trisomy 13). Fetal mortality was directly dependent on the presence of extra-CNS anomalies. Five fetuses ultimately died, including all four with multiple extra-CNS anomalies. The remaining two patients are alive at 1.5 and 6 years of age. We conclude that DWM has a distinctive sonographic appearance that can be identified by prenatal ultrasound and that identification of DWM should initiate a careful search for concurrent anomalies as well as consideration of chromosomal analysis.