Dilatation of the fetal cerebral ventricles (ventriculomegaly) is a generic sonographic sign that is common to several pathological entities carrying different prognoses. The main causes of fetal ventriculomegaly are aqueductal stenosis, Chiari II malformation, Dandy-Walker complex, and agenesis of the corpus callosum. Ventriculomegaly is easily recognized by ultrasound by measuring the atrial width. This simple measure allows the recognition of mild forms of ventricular dilatation and is used in screening for ventriculomegaly. However, although the diagnosis of ventriculomegaly is easy, the prenatal identification of the cause of ventricular dilatation is a more difficult task. For this purpose the evaluation of the posterior fossa in association with the visualization of the corpus callosum is useful. Research into the causes of ventriculomegaly is clinically useful, since the prognosis mainly depends on the etiology and on the presence of associated abnormalities. In this article the role of prenatal sonography in determining the cause of the ventriculomegaly is reviewed, as well as the prognostic value of the prenatal sonographic findings.