OBJECTIVE To obtain a nomogram of the clivus-supraocciput angle as a basis for the diagnosis of Chiari II malformation in fetuses with ventriculomegaly. METHODS A cross-sectional study was undertaken on 310 normal pregnant women of 16-34 weeks' gestation. A mid-sagittal section of the fetal skull was obtained and the angle between the clivus and the supraocciput was measured. Forty-four fetuses with ventriculomegaly due to various causes (13 Chiari II malformation, 12 dysgenesis of the corpus callosum, 7 aqueductal stenosis, 6 borderline ventriculomegaly, 3 Dandy-Walker malformation, 2 porencephaly, 1 schizencephaly) were also included in the study and the values of the angle found in the pathological cases were compared with those found in the normal population. RESULTS The clivus-supraocciput angle did not change during gestation and was almost constant with an average value of 79.3 +/- 6 degrees. All cases of Chiari II malformation showed a value below the 5th centile of our nomogram. CONCLUSIONS The evaluation of the posterior fossa and particularly the measurement of the clivus-supraocciput angle is a useful parameter to differentiate the various causes of fetal ventriculomegaly and particularly to recognize Chiari II malformation.