Thirteen cases of documented ventriculomegaly were diagnosed between 19 and 24 weeks of gestation. In 11 cases, the lateral ventricle/hemispheric width ratio was abnormally increased at the time of the first ultrasound examination, and in two, the lateral ventricle/hemispheric width ratios were initially in the high normal range. Severe ventriculomegaly was observed in both of the latter cases when repeat examinations were performed two weeks later. In 12 cases (92%), the biparietal diameter (BPD) was appropriate for gestational age, whereas the BPD was increased in only one. Two of the affected fetuses were found in conjunction with normal twins. This series suggests that fetal ventriculomegaly can be accurately diagnosed before fetal viability using the lateral ventricle/hemispheric width ratio.