Fourteen cases of holoprosencephaly (HP), including 10 cases of alobar HP and four cases of semilobar HP, were identified by prenatal sonography. Intracranial and extracranial findings were reviewed to determine the accuracy and spectrum of the sonographic features. All 14 cases were reliably distinguished from other causes of fetal hydrocephalus (n = 58) detected during the same period by demonstrating absence of the midline echo (falx cerebri and interhemispheric fissure), fusion of the thalami, and abnormal ventricular configuration. Four cases of semilobar HP demonstrated incomplete fusion of the thalami and partial separation of the ventricles compared with alobar HP. Eight cases demonstrated a dorsal cyst including five with alobar HP and three with semilobar HP. One case demonstrated an unusual extraaxial fluid collection surrounding the brain, thought to be from rupture of a dorsal cyst. Facial features that were identified included a proboscis (three cases), midline facial cleft (three cases), and hypotelorism (five cases). Extracranial abnormalities that were identified included polydactyly (two cases), renal dysplasia (two cases), omphalocele (one case), and fetal hydrops (one case). We conclude that fetuses with HP can exhibit a spectrum of sonographic findings and that alobar or semilobar HP is reliably distinguished from other causes of fetal hydrocephalus by distinctive intracranial findings.