The ultrasonograms of 15 patients with moderate to marked fetal hydrocephalus were evaluated retrospectively in an attempt to determine the effect of ventricular dilatation upon biometry of the fetal head in the latter half of pregnancy. In 13 of the 15 cases the age based upon biometry of the fetal head was within one week of the clinical estimate of gestational age based upon the last menstrual period. As accurate head measurements may be useful even in cases of hydrocephalus, these measurements should not be dismissed as being invalid particularly when commensurate with a clinical estimate of gestational age.